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diff --git a/Source/Initialization/InjectorFlux.H b/Source/Initialization/InjectorFlux.H
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+++ b/Source/Initialization/InjectorFlux.H
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+/* Copyright 2023 Remi Lehe
+ *
+ * This file is part of WarpX.
+ *
+ * License: BSD-3-Clause-LBNL
+ */
+#ifndef INJECTOR_FLUX_H_
+#define INJECTOR_FLUX_H_
+
+#include "Utils/WarpXConst.H"
+
+#include <AMReX.H>
+#include <AMReX_Array.H>
+#include <AMReX_GpuQualifiers.H>
+#include <AMReX_Math.H>
+#include <AMReX_Parser.H>
+#include <AMReX_REAL.H>
+
+#include <cmath>
+#include <string>
+
+// struct whose getFlux returns constant flux.
+struct InjectorFluxConstant
+{
+    InjectorFluxConstant (amrex::Real a_flux) noexcept : m_flux(a_flux) {}
+
+    AMREX_GPU_HOST_DEVICE
+    amrex::Real
+    getFlux (amrex::Real, amrex::Real, amrex::Real, amrex::Real) const noexcept
+    {
+        return m_flux;
+    }
+
+private:
+    amrex::Real m_flux;
+};
+
+// struct whose getFlux returns local flux computed from parser.
+struct InjectorFluxParser
+{
+    InjectorFluxParser (amrex::ParserExecutor<4> const& a_parser) noexcept
+        : m_parser(a_parser) {}
+
+    AMREX_GPU_HOST_DEVICE
+    amrex::Real
+    getFlux (amrex::Real x, amrex::Real y, amrex::Real z, amrex::Real t) const noexcept
+    {
+        return m_parser(x,y,z,t);
+    }
+
+    amrex::ParserExecutor<4> m_parser;
+};
+
+// Base struct for flux injector.
+// InjectorFlux contains a union (called Object) that holds any one
+// instance of:
+// - InjectorFluxConstant  : to generate constant flux;
+// - InjectorFluxParser    : to generate flux from parser;
+// The choice is made at runtime, depending in the constructor called.
+// This mimics virtual functions.
+struct InjectorFlux
+{
+    // This constructor stores a InjectorFluxConstant in union object.
+    InjectorFlux (InjectorFluxConstant* t, amrex::Real a_flux)
+        : type(Type::constant),
+          object(t,a_flux)
+    { }
+
+    // This constructor stores a InjectorFluxParser in union object.
+    InjectorFlux (InjectorFluxParser* t, amrex::ParserExecutor<4> const& a_parser)
+        : type(Type::parser),
+          object(t,a_parser)
+    { }
+
+    // Explicitly prevent the compiler from generating copy constructors
+    // and copy assignment operators.
+    InjectorFlux (InjectorFlux const&) = delete;
+    InjectorFlux (InjectorFlux&&) = delete;
+    void operator= (InjectorFlux const&) = delete;
+    void operator= (InjectorFlux &&) = delete;
+
+    void clear ()
+    {
+        switch (type)
+        {
+        case Type::constant:
+        case Type::parser:
+        {
+            break;
+        }
+        }
+    }
+
+    // call getFlux from the object stored in the union
+    // (the union is called Object, and the instance is called object).
+    AMREX_GPU_HOST_DEVICE
+    amrex::Real
+    getFlux (amrex::Real x, amrex::Real y, amrex::Real z, amrex::Real t) const noexcept
+    {
+        switch (type)
+        {
+        case Type::parser:
+        {
+            return object.parser.getFlux(x,y,z,t);
+        }
+        case Type::constant:
+        {
+            return object.constant.getFlux(x,y,z,t);
+        }
+        default:
+        {
+            amrex::Abort("InjectorFlux: unknown type");
+            return 0.0;
+        }
+        }
+    }
+
+private:
+    enum struct Type { constant, parser };
+    Type type;
+
+    // An instance of union Object constructs and stores any one of
+    // the objects declared (constant or parser).
+    union Object {
+        Object (InjectorFluxConstant*, amrex::Real a_flux) noexcept
+            : constant(a_flux) {}
+        Object (InjectorFluxParser*, amrex::ParserExecutor<4> const& a_parser) noexcept
+            : parser(a_parser) {}
+        InjectorFluxConstant   constant;
+        InjectorFluxParser     parser;
+    };
+    Object object;
+};
+
+// In order for InjectorFlux to be trivially copyable, its destructor
+// must be trivial.  So we have to rely on a custom deleter for unique_ptr.
+struct InjectorFluxDeleter {
+    void operator () (InjectorFlux* p) const {
+        if (p) {
+            p->clear();
+            delete p;
+        }
+    }
+};
+
+#endif