什么是“<？=”运算符在C ++？ [英] What's "<?=" operator in C++?

问题描述

我遇到了以下代码这里，这是来自使用邻接的Dijkstra算法的C ++实现

I came across the following code here, which is from the C++ implementation of Dijkstra algorithm using an adjacency matrix.

//read in edges keeping only the minimum
for(int i=0; i<E; i++) {
    int v1,v2,tmp;
    fin >> v1; fin >> v2;
    fin >> tmp;
    adjmat[v1][v2]<?=tmp; // <?= sets left to min(left,right)
    adjmat[v2][v1]<?=tmp;
}

注意最后两行，应用运算符 <？= 。正在评论，以下行

Pay attention to the last two lines, which apply operator <?=. As being commented, the following line

adjmat [v1] [v2]<？= tmp; //<？= set left to min（left，right）

会设置 left 到 min（left，right）。

我试过在VS中的代码，它不能编译。它是什么？如何设置 left 为 min（left，right）？

I never see this operator before. I tried the code in VS, it can not compile. What is it? How can it set left to be min(left,right)?

推荐答案

这是一个旧的 GCC extension ;它做它在注释中说的（它是最小运算符的复合赋值形式）。这不是标准的C ++。

It is an old GCC extension; it does what it says in the comment (it's the compound assignment form of the "minimum" operator). This is not standard C++.

a = a< b？ a：b 和 a<？= b 是后者只评估每个操作数一次。

The difference between a = a < b ? a : b and a <?= b is that the latter only evaluates each operand once.

在现代标准C ++中，我相信你可以这样写一个赋值最小算法：

In modern standard C++, I believe you could write an "assign the minimum" algorithm like this:

auto && __a = a;
auto && __b = b;
if (!(__a < __b)) { __a = std::forward<decltype(__b)>(__b); }

这应该是一个宏的主体，它的效果是 a <？= b 和 a 和 b 是任意表达式，效果。或者您可以将其封装到模板中：

This should be the body of a macro which has the effect of a <?= b, and a and b are arbitrary expressions, potentially with side effects. Or you could wrap it into a template:

template <typename T,
          typename U,
          typename P = std::less<std::common_type_t<std::decay_t<T>, std::decay_t<U>>>
T && clamp_to_minimum(T && a, U && b, P p = P())
{
    if (!(p(a, b))) { a = std::forward<U>(b); }
    return std::forward<T>(a);
}

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