cout 语句中的 运行 是什么？ (C++17)

Question

比如说我有一个很长的语句，比如

cout << findCurrent() << "," << findLowest() << "," << findHighest() << "," << findThird()<<"\n";

findCurrent() 会像逻辑指示的那样在 findLowest() 之前 运行 吗？

Answer 1

自 C++17 起，函数保证从左到右调用，即先调用 findCurrent()，然后调用 findLowest()，依此类推。

C++17标准参考：[expr.shift]/4（指表达式E1 << E2）：

The expression E1 is sequenced before the expression E2.

[over.match.oper]/2:（描述重载运算符）

the operands are sequenced in the order prescribed for the built-in operator.

[intro.execution]/15:

An expression X is said to be sequenced before an expression Y if every value computation and every side effect associated with the expression X is sequenced before every value computation and every side effect associated with the expression Y.

Link to cppreference summary

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在 C++17 之前，函数调用的顺序是 未指定的，这意味着它们可以以任何顺序调用（并且这个顺序不需要与重复调用）。

Answer 2

在 C++17 之前，the order of evaluation is unspecified。

从 C++17 开始，它需要从左到右计算。标准报价见。