是否可以知道参数是否为默认值 [英] Is it possible to know if the parameter was defaulted

问题描述

警告：此问题仅限于MSVS

我有此函数签名：

void do_somthing(std::vector<foo>&  bar={});

这两个调用函数可能不同：

Is it possible to differ between those two calls for the function:

第一个：

do_something()

第二：

std::vector<foo> v;
do_something(v);

换句话说，我想要的是：

In other words, I want something like:

void do_somthing(std::vector<foo>&  bar={}){
    if(/* bar was defaulted*/){

    }
    else{

    }
}

EDIT：
实际代码：

The actual code:

template<class Tinput_iterator>
            Tmodel perform_fitting(Tinput_iterator begin_data, Tinput_iterator end_data, std::vector<Tpoint>& inliers = {});

推荐答案

签名：

I have this function signature:

void do_somthing(std::vector<foo>&  bar=std::vector<foo>{});

此无法编译危险的非标准编译器设置你应该远离。

This cannot compile, except with dangerous non-standard compiler settings you should stay away from.

特别是，Visual C ++允许这个如果 / Za 未指定，但 / W4 仍会产生如下警告：

In particular, Visual C++ allows this if /Za is not specified, but with /W4 still produces a warning like this:

stackoverflow.cpp(6): warning C4239: nonstandard extension used: 'default argument': conversion from 'std::vector<foo,std::allocator<_Ty>>' to 'std::vector<foo,
std::allocator<_Ty>> &'
        with
        [
            _Ty=foo
        ]
stackoverflow.cpp(6): note: A non-const reference may only be bound to an lvalue

void do_somthing(std::vector<foo>&  bar=std::vector<foo>{}){
    if(/* bar was defaulted*/){

    }
    else{

    }
}

即使我们假设你实际上包含了缺少的 const 来编译代码，答案是：不，不可能知道 bar 是否为默认值。

Even if we assume that you actually included the missing const to make the code compile, the answer would be: no, it is not possible to know if bar was defaulted.

无论您打算在这里做什么，都必须找到完全不同的解决方案。

Whatever you plan to do here, you have to find a completely different solution.

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