具有模板专业化的泛型类型转换

Question

我正在努力编写一个用于类型转换的通用接口。我希望通过模板专门化来实现。我的想法是要有一个基本的模板实现来引发异常，即在没有专门化的情况下。用户应该提供所有可能的转换。

我有一个小的实现，当然是行不通的:)

#include <iostream>
#include <functional>

template <typename T, typename F>
T convert(const F & from)
{
    std::cout << __PRETTY_FUNCTION__ << std::endl;
    throw ( std::logic_error("Conversion not implemented!") );
}

template<>
int convert<int, std::string  >( const std::string &  from) {
    return 1;
}

int main() {
    int a;
    std::string b = "hello world";
    const std::string &br = b;
    a = convert<decltype(a), std::remove_reference_t<std::remove_const_t<decltype(br)>> >(br);

    std::cout << a << std::endl;
    return 0;
}

我不明白为什么上面的代码无法正常工作。我正在从中删除const和reference br，因此它应该调用已实现的专业化，但不是。我期待您的积极答复，也希望有一种更有效的方法来调用convert API，而无需在模板参数中指定类型。

此致-Adnan

Answer 1

I am removing const and reference from the br

Yes, but in the wrong order.

You remove before const and after reference; you should remove reference before and const after

// ......................................................VVVVVVVVV  reference before
a = convert<decltype(a), std::remove_const_t<std::remove_reference_t<decltype(br)>> >(br);
// ..................................^^^^^   const after

The point is that decltype(br) is std::string const &; I mean: const is applied only to std::string, not to std::string &.

On the contrary, the reference is applied to std::string const.

So when you apply std::remove_const to std::string const &, you get again std::string const &, because the full type isn't const.

Next you apply std::remove_reference to std::string const & and you get std::string const.

So the const remain.

If you use first std::remove_reference, from std::string const & you obtain std::string const.

Then you use std::remove_const (this time works, because the full type is const) and you get std::string.