diff --git a/docs/source/io_formats/settings.rst b/docs/source/io_formats/settings.rst
index 6bd5c3bc1..33674ca32 100644
--- a/docs/source/io_formats/settings.rst
+++ b/docs/source/io_formats/settings.rst
@@ -462,25 +462,23 @@ attributes/sub-elements:
   :library:
     If this attribute is given, it indicates that the source is to be
     instantiated from an externally compiled source function. This source can be
-    as complex as is required to define the source for your problem. The only
-    requirement is that there is a function called ``sample_source()``. More
-    documentation on how to build sources can be found in :ref:`custom_source`.
+    as complex as is required to define the source for your problem. The library
+    has a few basic requirements:
+
+    * It must contain a class that inherits from ``openmc::CustomSource``;
+    * The class must implement a function called ``sample_source()``;
+    * There must be a ``create_openmc_source()`` function that creates the source
+      as a unique pointer. This function can be used to pass parameters through to
+      the source from the XML, if needed.
+
+    More documentation on how to build sources can be found in :ref:`custom_source`.
 
     *Default*: None
 
   :parameters:
-    If this attribute is given, it indicates that the source is to be
-    instantiated from an externally compiled source function, with parameters
-    defined by the string provided in this attribute. In this case, the
-    custom source library must define a class that inherits from the
-    ``openmc::CustomSource`` abstract class. This class must implement a
-    ``sample_source()`` function, which takes an unsigned integer pointer as an
-    argument. The custom source library must also contain an
-    ``openmc_create_source`` method, which takes the value provided to the
-    parameters attribute as an argument and returns a pointer to an instance of
-    the custom source. If the library attribute is not provided then this
-    attribute will be ignored. More documentation on how to build parametrized
-    sources can be found in :ref:`parameterized_custom_source`.
+    If this attribute is given, it provides the parameters to pass through to the
+    class generated using the ``library`` parameter . More documentation on how to
+    build parametrized sources can be found in :ref:`parameterized_custom_source`.
 
     *Default*: None
 
diff --git a/docs/source/usersguide/settings.rst b/docs/source/usersguide/settings.rst
index 30bf71132..b992ed2ae 100644
--- a/docs/source/usersguide/settings.rst
+++ b/docs/source/usersguide/settings.rst
@@ -182,42 +182,55 @@ Custom Sources
 
 It is often the case that one may wish to simulate a complex source distribution
 that is not possible to represent with the classes described above. For these
-situations, it is possible to define a complex source with an externally defined
-source function that is loaded at runtime. A simple example source is shown
+situations, it is possible to define a complex source class containing an externally
+defined source function that is loaded at runtime. A simple example source is shown
 below.
 
 .. code-block:: c++
 
-   #include "openmc/random_lcg.h"
-   #include "openmc/source.h"
-   #include "openmc/particle.h"
+  #include <memory> // for unique_ptr
 
-   // you must have external C linkage here
-   extern "C" openmc::Particle::Bank sample_source(uint64_t* seed) {
-     openmc::Particle::Bank particle;
-     // weight
-     particle.particle = openmc::Particle::Type::neutron;
-     particle.wgt = 1.0;
-     // position
-     double angle = 2.0 * M_PI * openmc::prn(seed);
-     double radius = 3.0;
-     particle.r.x = radius * std::cos(angle);
-     particle.r.y = radius * std::sin(angle);
-     particle.r.z = 0.0;
-     // angle
-     particle.u = {1.0, 0.0, 0.0};
-     particle.E = 14.08e6;
-     particle.delayed_group = 0;
-     return particle;
+  #include "openmc/random_lcg.h"
+  #include "openmc/source.h"
+  #include "openmc/particle.h"
+
+  class Source : public openmc::CustomSource
+  {
+    openmc::Particle::Bank sample_source(uint64_t* seed)
+    {
+      openmc::Particle::Bank particle;
+      // weight
+      particle.particle = openmc::Particle::Type::neutron;
+      particle.wgt = 1.0;
+      // position
+      double angle = 2.0 * M_PI * openmc::prn(seed);
+      double radius = 3.0;
+      particle.r.x = radius * std::cos(angle);
+      particle.r.y = radius * std::sin(angle);
+      particle.r.z = 0.0;
+      // angle
+      particle.u = {1.0, 0.0, 0.0};
+      particle.E = 14.08e6;
+      particle.delayed_group = 0;
+      return particle;
+    }
+  };
+
+  extern "C" std::unique_ptr<Source> openmc_create_source() {
+    return std::unique_ptr<Source> (new Source());
   }
 
 The above source creates monodirectional 14.08 MeV neutrons that are distributed
 in a ring with a 3 cm radius. This routine is not particularly complex, but
 should serve as an example upon which to build more complicated sources.
 
-  .. note:: The function signature must be declared ``extern "C"``.
+  .. note:: The source class must inherit from ``openmc::CustomSource`` and
+            implement a ``sample_source()`` function.
 
-  .. note:: You should only use the openmc::prn() random number generator
+  .. note:: The ``openmc_create_source()`` function signature must be declared
+            ``extern "C"``.
+
+  .. note:: You should only use the ``openmc::prn()`` random number generator.
 
 In order to build your external source, you will need to link it against the
 OpenMC shared library. This can be done by writing a CMakeLists.txt file:
@@ -240,61 +253,53 @@ used for sampling source particles at runtime.
 Custom Parameterized Sources
 ----------------------------
 
-If the custom source may be used with parameters at a variety of values then it
-may be necessary to represent those parameters as a string in order to avoid
-recompiling the source library for each run. This is supported by defining a
-class inheriting from ``openmc::CustomSource`` that implements a
-``sample_source`` function:
+Some custom sources may have values (parameters) that can be changed between
+runs. This is supported by using the ``create_custom_source()`` function to
+pass parameters defined in the :attr:`openmc.Source.parameters` attribute to
+the source class when it is created:
 
 .. code-block:: c++
 
+  #include <memory> // for unique_ptr
+
   #include "openmc/source.h"
   #include "openmc/particle.h"
 
-  class ParameterizedSource : public openmc::CustomSource {
-    public:
-      double energy;
-      
-      ParameterizedSource(double energy) {
-        this->energy = energy;
-      }
+  class Source : public openmc::CustomSource
+  {
+    double energy;
 
-      // Samples from an instance of this class.
-      openmc::Particle::Bank sample_source(uint64_t* seed) {
-        openmc::Particle::Bank particle;
-        // wgt
-        particle.particle = openmc::Particle::Type::neutron;
-        particle.wgt = 1.0;
-        // position
-        particle.r.x = 0.0;
-        particle.r.y = 0.0;
-        particle.r.z = 0.0;
-        // angle
-        particle.u = {1.0, 0.0, 0.0};
-        particle.E = this->energy;
-        particle.delayed_group = 0;
+    Source(double energy)
+    {
+      this->energy = energy;
+    }
 
-        return particle;
-      }
+    // Samples from an instance of this class.
+    openmc::Particle::Bank sample_source(uint64_t* seed)
+    {
+      openmc::Particle::Bank particle;
+      // weight
+      particle.particle = openmc::Particle::Type::neutron;
+      particle.wgt = 1.0;
+      // position
+      particle.r.x = 0.0;
+      particle.r.y = 0.0;
+      particle.r.z = 0.0;
+      // angle
+      particle.u = {1.0, 0.0, 0.0};
+      particle.E = this->energy;
+      particle.delayed_group = 0;
+
+      return particle;
+    }
   };
 
-The custom source library function in this case must also define an
-``openmc_create_source`` method, which will be used to generate an instance of
-the custom source, based on the value supplied in the
-:attr:``openmc.Source.parameters`` attribute. The
-``openmc_create_source`` method must be defined with ``extern "C"``:
-
-.. code-block:: c++
-
-  // you must have external C linkage here otherwise
-  // dlopen will not find the file
-  extern "C" ParameterizedSource* openmc_create_source(const char* parameter) {
-    return new ParameterizedSource(atof(parameter));
+  extern "C" std::unique_ptr<Source> openmc_create_source(const char* parameter) {
+    return std::unique_ptr<Source> (new Source(atof(parameter)));
   }
 
 As with the basic custom source functionality, the custom source library
-location must also be provided in the :attr:`openmc.Source.library`
-attribute.
+location must be provided in the :attr:`openmc.Source.library` attribute.
 
 ---------------
 Shannon Entropy
diff --git a/examples/custom_source/source_ring.cpp b/examples/custom_source/source_ring.cpp
index d68122dd7..5b7bb837c 100644
--- a/examples/custom_source/source_ring.cpp
+++ b/examples/custom_source/source_ring.cpp
@@ -1,26 +1,36 @@
 #include <cmath> // for M_PI
+#include <memory> // for unique_ptr
 
 #include "openmc/random_lcg.h"
 #include "openmc/source.h"
 #include "openmc/particle.h"
 
-// you must have external C linkage here otherwise
-// dlopen will not find the file
-extern "C" openmc::Particle::Bank sample_source(uint64_t* seed)
+class Source : public openmc::CustomSource
 {
-  openmc::Particle::Bank particle;
-  // wgt
-  particle.particle = openmc::Particle::Type::neutron;
-  particle.wgt = 1.0;
-  // position
-  double angle = 2. * M_PI * openmc::prn(seed);
-  double radius = 3.0;
-  particle.r.x = radius * std::cos(angle);
-  particle.r.y = radius * std::sin(angle);
-  particle.r.z = 0.0;
-  // angle
-  particle.u = {1.0, 0.0, 0.0};
-  particle.E = 14.08e6;
-  particle.delayed_group = 0;
-  return particle;
+  openmc::Particle::Bank sample_source(uint64_t* seed)
+  {
+    openmc::Particle::Bank particle;
+    // wgt
+    particle.particle = openmc::Particle::Type::neutron;
+    particle.wgt = 1.0;
+    // position
+    double angle = 2.0 * M_PI * openmc::prn(seed);
+    double radius = 3.0;
+    particle.r.x = radius * std::cos(angle);
+    particle.r.y = radius * std::sin(angle);
+    particle.r.z = 0.0;
+    // angle
+    particle.u = {1.0, 0.0, 0.0};
+    particle.E = 14.08e6;
+    particle.delayed_group = 0;
+    return particle;
+  }
+};
+
+// A function to create a unique pointer to an instance of this class when generated
+// via a plugin call using dlopen/dlsym.
+// You must have external C linkage here otherwise dlopen will not find the file
+extern "C" std::unique_ptr<Source> openmc_create_source()
+{
+  return std::unique_ptr<Source> (new Source());
 }
diff --git a/examples/parameterized_custom_source/parameterized_source_ring.cpp b/examples/parameterized_custom_source/parameterized_source_ring.cpp
index 51218e1b3..abcaa975a 100644
--- a/examples/parameterized_custom_source/parameterized_source_ring.cpp
+++ b/examples/parameterized_custom_source/parameterized_source_ring.cpp
@@ -1,17 +1,20 @@
 #include <cmath> // for M_PI
+#include <memory> // for unique_ptr
 #include <unordered_map>
 
 #include "openmc/random_lcg.h"
 #include "openmc/source.h"
 #include "openmc/particle.h"
 
-class ParameterizedSource : public openmc::CustomSource {
+class Source : public openmc::CustomSource
+{
   protected:
     double radius_;
     double energy_;
 
-    // Protect the constructor so that the class can only be created by serialisation.
-    ParameterizedSource(double radius, double energy) {
+    // Protect the constructor as we only want the class to be created by the from_string method.
+    Source(double radius, double energy)
+    {
       radius_ = radius;
       energy_ = energy;
     }
@@ -21,9 +24,10 @@ class ParameterizedSource : public openmc::CustomSource {
     double radius() { return radius_; }
     double energy() { return energy_; }
 
-    // Defines a function that can create a pointer to a new instance of this class
-    // by deserializing from the provided string.
-    static ParameterizedSource* from_string(const char* parameters) {
+    // Defines a function that can create a unique pointer to a new instance of this class
+    // by extracting the parameters from the provided string.
+    static std::unique_ptr<Source> from_string(const char* parameters)
+    {
       std::unordered_map<std::string, std::string> parameter_mapping;
 
       std::stringstream ss(parameters);
@@ -35,17 +39,20 @@ class ParameterizedSource : public openmc::CustomSource {
         parameter_mapping[key] = value;
       }
 
-      return new ParameterizedSource(std::stod(parameter_mapping["radius"]), std::stod(parameter_mapping["energy"]));
+      return std::unique_ptr<Source> (
+        new Source(std::stod(parameter_mapping["radius"]), std::stod(parameter_mapping["energy"]))
+      );
     }
 
     // Samples from an instance of this class.
-    openmc::Particle::Bank sample_source(uint64_t* seed) {
+    openmc::Particle::Bank sample_source(uint64_t* seed)
+    {
       openmc::Particle::Bank particle;
       // wgt
       particle.particle = openmc::Particle::Type::neutron;
       particle.wgt = 1.0;
       // position
-      double angle = 2. * M_PI * openmc::prn(seed);
+      double angle = 2.0 * M_PI * openmc::prn(seed);
       double radius = this->radius();
       particle.r.x = radius * std::cos(angle);
       particle.r.y = radius * std::sin(angle);
@@ -59,9 +66,10 @@ class ParameterizedSource : public openmc::CustomSource {
     }
 };
 
-// A function to create a pointer to an instance of this class when generated
+// A function to create a unique pointer to an instance of this class when generated
 // via a plugin call using dlopen/dlsym.
 // You must have external C linkage here otherwise dlopen will not find the file
-extern "C" ParameterizedSource* openmc_create_source(const char* parameters) {
-  return ParameterizedSource::from_string(parameters);
+extern "C" std::unique_ptr<Source> openmc_create_source(const char* parameters)
+{
+  return Source::from_string(parameters);
 }
diff --git a/include/openmc/source.h b/include/openmc/source.h
index 5bc2901b4..460e7eca3 100644
--- a/include/openmc/source.h
+++ b/include/openmc/source.h
@@ -66,7 +66,7 @@ class CustomSource {
     virtual Particle::Bank sample_source(uint64_t* seed) = 0;
 };
 
-typedef CustomSource* create_custom_source_t(const char* parameters);
+typedef std::unique_ptr<CustomSource> create_custom_source_t(const char* parameters);
 
 //==============================================================================
 // Functions
diff --git a/src/source.cpp b/src/source.cpp
index 673ee8804..7c40147ce 100644
--- a/src/source.cpp
+++ b/src/source.cpp
@@ -5,6 +5,7 @@
 #endif
 
 #include <algorithm> // for move
+#include <memory> // for unique_ptr
 
 #ifdef HAS_DYNAMIC_LINKING
 #include <dlfcn.h> // for dlopen, dlsym, dlclose, dlerror
@@ -45,11 +46,8 @@ std::vector<SourceDistribution> external_sources;
 namespace {
 
 void* custom_source_library;
-using sample_t = Particle::Bank (*)(uint64_t* seed);
-sample_t custom_source_function;
-
-std::string custom_source_parameters;
-CustomSource* custom_source;
+std::string custom_source_parameters = "";
+std::unique_ptr<CustomSource> custom_source;
 
 }
 
@@ -374,17 +372,11 @@ void load_custom_source_library()
   // reset errors
   dlerror();
 
-  if (custom_source_parameters.empty()) {
-    // get the function from the library
-    using sample_t = Particle::Bank (*)(uint64_t* seed);
-    custom_source_function = reinterpret_cast<sample_t>(dlsym(custom_source_library, "sample_source"));
-  } else {
-    // get the function to create the CustomSource from the library
-    create_custom_source_t* create_custom_source = (create_custom_source_t*) dlsym(custom_source_library, "openmc_create_source");
+  // get the function to create the CustomSource from the library
+  create_custom_source_t* create_custom_source = (create_custom_source_t*) dlsym(custom_source_library, "openmc_create_source");
 
-    // create a pointer to an instance of the CustomSource
-    custom_source = create_custom_source(custom_source_parameters.c_str());
-  }
+  // create a pointer to an instance of the CustomSource
+  custom_source = create_custom_source(custom_source_parameters.c_str());
 
   // check for any dlsym errors
   auto dlsym_error = dlerror();
@@ -401,9 +393,9 @@ void load_custom_source_library()
 
 void close_custom_source_library()
 {
-  if (custom_source) {
-    // delete the CustomSource if it exists
-    delete custom_source;
+  if (custom_source.get()) {
+    // Make sure the custom source is destroyed before we close it's libary.
+    custom_source.reset();
   }
 
 #ifdef HAS_DYNAMIC_LINKING
@@ -416,12 +408,8 @@ void close_custom_source_library()
 
 Particle::Bank sample_custom_source_library(uint64_t* seed)
 {
-  if (custom_source_parameters.empty()) {
-    return custom_source_function(seed);
-  } else {
-    // sample from the instance of the CustomSource
-    return custom_source->sample_source(seed);
-  }
+  // sample from the instance of the CustomSource
+  return custom_source->sample_source(seed);
 }
 
 void fill_source_bank_custom_source()
diff --git a/tests/regression_tests/source_dlopen/source_sampling.cpp b/tests/regression_tests/source_dlopen/source_sampling.cpp
index eaf8b74d9..529e3a547 100644
--- a/tests/regression_tests/source_dlopen/source_sampling.cpp
+++ b/tests/regression_tests/source_dlopen/source_sampling.cpp
@@ -1,11 +1,14 @@
 #include <iostream>
+#include <memory>
+
 #include "openmc/random_lcg.h"
 #include "openmc/source.h"
 #include "openmc/particle.h"
 
-// you must have external C linkage here otherwise 
-// dlopen will not find the file
-extern "C" openmc::Particle::Bank sample_source(uint64_t *seed) {
+class Source : openmc::CustomSource
+{
+  openmc::Particle::Bank sample_source(uint64_t *seed)
+  {
     openmc::Particle::Bank particle;
     // wgt
     particle.particle = openmc::Particle::Type::neutron;
@@ -20,4 +23,13 @@ extern "C" openmc::Particle::Bank sample_source(uint64_t *seed) {
     particle.E = 14.08e6;
     particle.delayed_group = 0;
     return particle;    
+  }
+};
+
+// A function to create a unique pointer to an instance of this class when generated
+// via a plugin call using dlopen/dlsym.
+// You must have external C linkage here otherwise dlopen will not find the file
+extern "C" std::unique_ptr<Source> openmc_create_source()
+{
+  return std::unique_ptr<Source> (new Source());
 }
diff --git a/tests/regression_tests/source_parameterized_dlopen/inputs_true.dat b/tests/regression_tests/source_parameterized_dlopen/inputs_true.dat
index a7462ae7f..f4a0eba73 100644
--- a/tests/regression_tests/source_parameterized_dlopen/inputs_true.dat
+++ b/tests/regression_tests/source_parameterized_dlopen/inputs_true.dat
@@ -19,5 +19,5 @@
   <particles>1000</particles>
   <batches>10</batches>
   <inactive>0</inactive>
-  <source library="build/libparameterized_source.so" parameters="energy=1e3" strength="1.0" />
+  <source library="build/libsource.so" parameters="1e3" strength="1.0" />
 </settings>
diff --git a/tests/regression_tests/source_parameterized_dlopen/parameterized_source_sampling.cpp b/tests/regression_tests/source_parameterized_dlopen/parameterized_source_sampling.cpp
index b3e9ca9cd..162164f01 100644
--- a/tests/regression_tests/source_parameterized_dlopen/parameterized_source_sampling.cpp
+++ b/tests/regression_tests/source_parameterized_dlopen/parameterized_source_sampling.cpp
@@ -1,16 +1,19 @@
 #include "openmc/source.h"
 #include "openmc/particle.h"
 
-class ParameterizedSource : public openmc::CustomSource {
+class Source : public openmc::CustomSource
+{
   public:
     double energy;
 
-    ParameterizedSource(double energy) {
+    Source(double energy)
+    {
       this->energy = energy;
     }
 
     // Samples from an instance of this class.
-    openmc::Particle::Bank sample_source(uint64_t* seed) {
+    openmc::Particle::Bank sample_source(uint64_t* seed)
+    {
       openmc::Particle::Bank particle;
       // wgt
       particle.particle = openmc::Particle::Type::neutron;
@@ -28,8 +31,10 @@ class ParameterizedSource : public openmc::CustomSource {
     }
 };
 
-// you must have external C linkage here otherwise
-// dlopen will not find the file
-extern "C" ParameterizedSource* openmc_create_source(const char* parameter) {
-  return new ParameterizedSource(atof(parameter));
+// A function to create a unique pointer to an instance of this class when generated
+// via a plugin call using dlopen/dlsym.
+// You must have external C linkage here otherwise dlopen will not find the file
+extern "C" std::unique_ptr<Source> openmc_create_source(const char* parameter)
+{
+  return std::unique_ptr<Source> (new Source(atof(parameter)));
 }
diff --git a/tests/regression_tests/source_parameterized_dlopen/test.py b/tests/regression_tests/source_parameterized_dlopen/test.py
index 6c88c37c2..c613cde4b 100644
--- a/tests/regression_tests/source_parameterized_dlopen/test.py
+++ b/tests/regression_tests/source_parameterized_dlopen/test.py
@@ -20,9 +20,9 @@ def compile_source(request):
         f.write(textwrap.dedent("""
             cmake_minimum_required(VERSION 3.3 FATAL_ERROR)
             project(openmc_sources CXX)
-            add_library(parameterized_source SHARED parameterized_source_sampling.cpp)
+            add_library(source SHARED parameterized_source_sampling.cpp)
             find_package(OpenMC REQUIRED HINTS {})
-            target_link_libraries(parameterized_source OpenMC::libopenmc)
+            target_link_libraries(source OpenMC::libopenmc)
             """.format(openmc_dir)))
 
     # Create temporary build directory and change to there
@@ -63,8 +63,8 @@ def model():
 
     # custom source from shared library
     source = openmc.Source()
-    source.library = 'build/libparameterized_source.so'
-    source.parameters = 'energy=1e3'
+    source.library = 'build/libsource.so'
+    source.parameters = '1e3'
     model.settings.source = source
 
     return model