如何取得 C 範本中的類型名稱？

Getting Type Name in C++ Templates

In C++ template programming, obtaining the name of the type being converted to can be crucial for error handling. Consider a template class parsing data files, where most parse errors originate from data file errors. To provide user-friendly error messages, identifying the expected data type is essential.

The generic GetValue function retrieves values from a data map based on section and key inputs. When the expected value cannot be converted to the desired type, an exception is thrown. However, determining the expected type name to include in the error message can be challenging.

Compile-Time Solution

An elegant compile-time solution can be achieved using type introspection and predefined macros. Create a header file (types.h) defining a function template GetTypeName.

template<typename T> const wchar_t *GetTypeName();

#define DEFINE_TYPE_NAME(type, name) \
    template<>const wchar_t *GetTypeName<type>(){return name;}

Use the DEFINE_TYPE_NAME macro in .cpp files to define the type names for types encountered in the data file parsing process.

DEFINE_TYPE_NAME(int, L"int")
DEFINE_TYPE_NAME(float, L"float")
DEFINE_TYPE_NAME(std::string, L"std::string")

Within the GetValue function, retrieve the type name using:

const wchar_t *typeName = GetTypeName<T>();

This approach ensures the creation of a linker error if an undefined type is encountered, prompting the addition of the necessary type definition.

Alternative Solution Using Runtime Type Identification (RTTI)

A runtime solution can be obtained using RTTI:

std::type_info(T).name()

However, this solution incurs runtime overhead, making it less desirable for frequently called functions.

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