Question

Question: I receive the following error for the code below, does anyone know why?

Problem:
I am working on a class (ClassB) that controls the behavior of a number of classes from an outside library (libMesh). The “…do something… portion of the code is designed to set some variables in these outside library classes that have template functions.

I would like to be able to set some of these values from the constructor of the inheriting class (ClassC). But, if I do this, as in the code below, I get the error shown. If I remove this command in the constructor it works just fine.

I also include a more detailed example that uses that produces the same error, but uses the libmesh class itself, it illustrates what I want to do a bit better. I am unsure of the usefulness of what I am trying to do, I mainly want to know why this doesn’t work because it seems like it should.

I found one other similar post, but I can’t seem to apply them to my problem.

Template inheritance inner class access problem

Thanks for the help,
Andrew

ERROR:

XXXXXXX@XXXXX:~/Documents/programs/build$ make test
[100%] Building CXX object CMakeFiles/test.dir/source/test.cpp.o
test.cpp: In constructor ‘ClassC<T>::ClassC()’:
test.cpp:16:29: error: expected primary-expression before ‘int’
test.cpp:16:29: error: expected ‘;’ before ‘int’
make[3]: *** [CMakeFiles/test.dir/source/test.cpp.o] Error 1
make[2]: *** [CMakeFiles/test.dir/all] Error 2
make[1]: *** [CMakeFiles/test.dir/rule] Error 2
make: *** [test] Error 2

SIMPLE CODE:

// A class that sets that sets the value of something 
template <typename Type> class ClassB{
public:
    ClassB(){}
    template<typename TypeValue> void set_value(TypeValue value){
        // ... do something ...
    }
};

// A class that inherits ClassB
template<typename T> class ClassC : public ClassB<T>{
public:
    ClassC(){
        // I want to do this (if I remove this it compiles) 
        ClassB<T>::set_value<int>(1);;
    }
};

// The main function
int main (){
    ClassC<double> c;
    c.set_value<int>(1); // This works
}

PROBLEM SPECIFIC CODE:

//! \example test_libmesh.cpp

#include <string>
using std::string;

// libMesh includes
#include <libmesh.h>
#include <libmesh_common.h> 
#include <equation_systems.h>
#include <transient_system.h>
#include <explicit_system.h>
#include <parameters.h>
#include <mesh.h>
using namespace libMesh;

// Fundamental behavior that will be used among many classes
template <typename Type> class EqCore{
    public:

        // Class constructor
        EqCore(EquationSystems& sys, string name) : eq_sys(sys){

            // Creates a system that will store the constant(s)
            name_.assign(name);
            eq_sys.add_system<Type>(name_); 

            // I can set stuff from here
            set_constant<double>("test4", 4);
        }

        // A function for storing a constant value
        template<typename ParamType> void set_constant(std::string name, ParamType var){  
            eq_sys.parameters.set<ParamType>(name) = var;
        }

        // A function for retrieving a constant value
        template<typename ParamType> ParamType get_constant(std::string name){  
            ParamType output = eq_sys.parameters.get<ParamType>(name);
            return output;
        }

        // Reference to the controlling equation system
        EquationSystems& eq_sys;    

        // The name of the system holding the constant(s)
        string name_;
};

// A test class derived
template <typename Type> class EqBase : public EqCore<Type>{
    public: 

        // Constructor
        EqBase(EquationSystems& sys, string name) : EqCore<Type>(sys, name){    

            // I want to do this!
            // (remove this and the associated print statement in the main and it works)
            EqCore<Type>::set_constant<double>("test5", 5);
        }   

};  

// Begin main function
int main (int argc, char** argv){

    // Initialize libMesh and create an empty mesh
    LibMeshInit init (argc, argv);
    Mesh mesh;

    // Test w/o any of the above classes
    EquationSystems eq_sys(mesh);
    eq_sys.parameters.set<double>("test1") = 1;
    printf("Test 1: %f\n", eq_sys.parameters.get<double>("test1"));

    // Test EqBase/EqCore functions set/get functions
    EqBase<TransientExplicitSystem> eq(eq_sys, "TestSystem");
    eq.set_constant<double>("test2", 2);
    printf("Test 2: %f\n", eq.get_constant<double>("test2"));

    // Test generic creation but accessed through EqBase
    eq.eq_sys.parameters.set<double>("test3") = 3;
    printf("Test 3: %f\n", eq.eq_sys.parameters.get<double>("test3"));

    // Test the constant created in EqCore constructor from EqBase
    printf("Test 4: %f\n", eq.eq_sys.parameters.get<double>("test4"));

    // Test the constant created in EqBase constructor from EqBase
    printf("Test 5: %f\n", eq.eq_sys.parameters.get<double>("test5"));

}

Answer 1

The compiler can’t work out how to parse this thing because it can’t work out that set_value is the name of a template. If you add the keyword template after the :: it fixes the problem:

ClassC(){
    // I want to do this (if I remove this it compiles) 
    ClassB<T>::template set_value<int>(1);;
}

The answer to this question goes into great detail about why:
Where and why do I have to put the "template" and "typename" keywords?