如何在 C++11 的模板中只接受数字和字符串？

Question

As titlte says, in C++11, how can I declare a template that only accepts numbers ( int , long , float and double ) and strings?

template<typename T>
class CustomClass {
    public:
        T data;
};

Answer 1

Put this anywhere in the class definition:

static_assert(std::is_arithmetic<T>::value ||
              std::is_same<T, std::string>::value,
              "Wrong argument type");

Adjust condition to taste.

Answer 2

By example, using template partial specialization and a template default value.

Something as

template <typename T, bool =    std::is_arithmetic<T>::value
                             || std::is_same<T, std::string>::value>
class CustomClass;

template <typename T>
class CustomClass<T, true>
 {
   public:
      T data;
 };

So you can have

CustomClass<int>  cci;
CustomClass<std::string>  ccs;
// CustomClass<std::istringstream>  cciss; // compilation error

Answer 3

I know question seems specific to C++11. But the language improved beautifully. One can use concepts from C++20. ( #include <concepts> ).

Use the requires keyword as shown below, and apply conditions.

requires std::integral<T> || std::floating_point<T> || std::is_convertible_v<T, std::string_view>

Example usage and testing:

//SAMPLE CLASS
class Sample
{

};

template<typename T>
//ALLOW INTEGER TYPES, FLOATING POINT TYPES, STRING TYPES.
requires std::integral<T> || std::floating_point<T> || std::is_convertible_v<T, std::string_view>
class CustomClass {
public:
    T data;
};

//ALLOWED
CustomClass<int> C_Int;
C_Int.data = 10;

//ALLOWED
CustomClass<std::string> C_Str;
C_Str.data = "Test";

//NOT ALLOWED UNLESS THE requires IS COMMENTED IN ABOVE CODE.
Sample s;
CustomClass<Sample> C_Sample;
C_Sample.data = s;