c++ - Boost Overload 奇怪的行为.. `int (int ), int (std::string )` 与 `int (int ), int (std::string ), std::string (std::string)` 有何不同？

Question

所以有一个很棒的库叫做 OverLoad (link to downloadable svn directory, lib is header only) .它可以接受任何类型的函数并自动决定你调用的是哪一个。它就像 boost 功能，但更好。这里有2个代码示例(浏览器可以查看boost svn)one two .这是我的代码，不编译并基于它们:

#include <string>

#include <boost/detail/lightweight_test.hpp>

#include <boost/overload.hpp>

using boost::overload; 

template<class out, class in>
out foo(in O )
{
    std::cout << "yes we can!";
    return out();
}

int main()
{
    //// works
    //overload<int (int ), int (std::string )> f;
    //// works
    //int (*foo1) (int ) = &foo<int, int>;
    //int (*foo2) (std::string ) = &foo<int, std::string>;
    //f.set(foo1);
    //f.set(foo2);
    // or we can use
    //// does also work
    //f.set<int (int )>(&foo<int, int>);
    //f.set<int (std::string )>(&foo<int, std::string>);
    ////

    overload<int (int ), int (std::string ), std::string (std::string) > f;
    //// but when we do this
    //f.set<int (int )>(&foo<int, int>);
    //f.set<int (std::string )>(&foo<int, std::string>);
    //f.set<int (std::string )>(&foo<std::string, std::string>);
    //// or this:
    int (*foo1) (int ) = &foo<int, int>;
    int (*foo2) (std::string ) = &foo<int, std::string>;
    std::string (*foo3) (std::string ) = &foo<std::string, std::string>;
    f.set(foo1);
    f.set(foo2);
    f.set(foo3);
    //// we get compile error

    BOOST_ASSERT( f(0) == 1 );
    BOOST_ASSERT( f("hi") == 2 ); // here we get Error  1   error C3066: there are multiple ways that an object of this type can be called with these arguments

    return boost::report_errors();
}

所以我想知道如何解决这个问题？

Answer 1

重载解析只考虑参数类型；它不考虑返回类型。因此，在重载解析期间，int (std::string) 与 std::string(std::string) 没有区别。

由于这个库必须依赖于 C++ 语言的重载能力，所以它也无法区分这两个函数。