实现编译时的“ static-if”。容器中不同字符串类型的逻辑 [英] Implementing a compile-time "static-if" logic for different string types in a container

问题描述

我想编写一个在字符串容器上运行的函数模板，例如 std :: vector 。

I'd like to write a function template that operates on a container of strings, for example a std::vector.

我想使用相同的模板功能同时支持 CString 和 std :: wstring

I'd like to support both CString and std::wstring with the same template function.

问题是 CString 和wstring具有不同的接口，例如，获取a的长度 CString ，则调用 GetLength（）方法，而对于wstring，则调用 size（） 或 length（）。

The problem is that CString and wstring have different interfaces, for example to get the "length" of a CString, you call the GetLength() method, instead for wstring you call size() or length().

如果我们有静态 功能在C ++中，我可以这样写：

If we had a "static if" feature in C++, I could write something like:

template <typename ContainerOfStrings>
void DoSomething(const ContainerOfStrings& strings)
{
    for (const auto & s : strings)
    {
        static_if(strings::value_type is CString) 
        {
            // Use the CString interface
        }
        static_else_if(strings::value_type is wstring)
        {   
            // Use the wstring interface
        }
    }
}

是否有一些模板编程技术可以实现这一目标使用当前可用的C ++ 11/14工具的目标是什么？

Is there some template programming technique to achieve this goal with currently available C++11/14 tools?

PS

我知道可以写一些的 DoSomething（）重载带有 vector< CString> 和 vector< wstring> ，但这不是问题的重点。

而且，我希望此函数模板适用于您可以使用range-for循环进行迭代的任何容器。

PS
I know it's possible to write a couple of DoSomething() overloads with vector<CString> and vector<wstring>, but that's not the point of the question.
Moreover, I'd like this function template to work for any container on which you can iterate using a range-for loop.

推荐答案

#include <type_traits>

template <typename T, typename F>
auto static_if(std::true_type, T t, F f) { return t; }

template <typename T, typename F>
auto static_if(std::false_type, T t, F f) { return f; }

template <bool B, typename T, typename F>
auto static_if(T t, F f) { return static_if(std::integral_constant<bool, B>{}, t, f); }

template <bool B, typename T>
auto static_if(T t) { return static_if(std::integral_constant<bool, B>{}, t, [](auto&&...){}); }

测试：

template <typename ContainerOfStrings>
void DoSomething(const ContainerOfStrings& strings)
{
    for (const auto & s : strings)
    {
        static_if<std::is_same<typename ContainerOfStrings::value_type, CString>{}>
        ([&](auto& ss)
        {
            // Use the CString interface
            ss.GetLength();
        })(s);

        static_if<std::is_same<typename ContainerOfStrings::value_type, wstring>{}>
        ([&](auto& ss)
        {
            // Use the wstring interface
            ss.size();
        })(s);
    }
}

演示

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