在C ++中使用模板成员函数继承模板类

问题描述

问题：我收到以下代码的以下错误，有人知道原因吗？

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

问题：
我正在研究一个类（ClassB），它控制来自外部库的许多类的行为（）。 ......做某事......代码的一部分用于在这些具有模板函数的外部库类中设置一些变量。

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.

我想成为能够从继承类（ClassC）的构造函数中设置一些这些值。但是，如果我这样做，就像在下面的代码中，我得到错误。如果我在构造函数中删除此命令它工作得很好。

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.

我还包含一个更详细的例子，使用它会产生相同的错误，但使用类本身，它说明了我想要做得更好一些。我不确定我想做什么的有用性，我主要想知道为什么这不起作用因为它似乎应该。

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.

我发现了另外一篇类似的帖子，但我似乎无法将它们应用到我的问题中。

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

感谢您的帮助，
Andrew

Thanks for the help,Andrew

错误：

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 ：

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
}

问题特定代码：

//! \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"));

}

推荐答案

编译器无法弄清楚如何解析这个东西，因为它无法解决set_value是模板的名称。如果您在 :: 之后添加关键字模板，则可以解决问题：

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);;
}

这个问题的答案详细解释了原因：

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

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