c++ - 为什么不调用移动构造函数？

Question

为什么在此示例中没有打印任何内容？我在 Coliru 上用 Clang 编译。

#include <iostream>

struct S
{
    S()    noexcept = default;
    S(S&&) noexcept { std::cout << "move-ctor"; }
};

void f(S) {}

int main()
{
    f(S{});
}

Answer 1

编译器正在执行复制省略，这是 C++11 标准第 12.8/31 段所允许的，即使您的移动构造函数、复制构造函数或析构函数有副作用也是如此:

When certain criteria are met, an implementation is allowed to omit the copy/move construction of a class object, even if the constructor selected for the copy/move operation and/or the destructor for the object have side effects.

即使在省略移动时也使用术语复制省略:

This elision of copy/move operations, called copy elision, is permitted in the following circumstances (which may be combined to eliminate multiple copies):

[...]

— when a temporary class object that has not been bound to a reference (12.2) would be copied/moved to a class object with the same cv-unqualified type, the copy/move operation can be omitted by constructing the temporary object directly into the target of the omitted copy/move

[...]

使用 GCC，您可以使用 -fno-elide-constructors 来禁止复制省略。在这种情况下，您会看到移动构造函数被调用，如 live example 所示。 .