移动由逗号运算符抑制的构造函数
这个程序:
#include <iostream>
struct T {
T() {}
T(const T &) { std::cout << "copy constructor "; }
T(T &&) { std::cout << "move constructor "; }
};
int main() {
([](T t) -> T { return t; })({}); std::cout << '
';
([](T t) -> T { return void(), t; })({}); std::cout << '
';
([](T t) -> T { return void(), std::move(t); })({}); std::cout << '
';
}
当由 gcc-4.7.1 编译时输出(link):
when compiled by gcc-4.7.1 outputs (link):
move constructor
copy constructor
move constructor
为什么逗号操作符会有这种效果?标准说:
Why does the comma operator have this effect? The standard says:
1 - [...] 类型结果的和值是右操作数的类型和值；结果与其右操作数[...]具有相同的值类别.如果右操作数的值是临时的，则结果是临时的.
5.18 Comma operator [expr.comma]
1 - [...] The type and value of the result are the type and value of the right operand; the result is of the same value category as its right operand [...]. If the value of the right operand is a temporary, the result is that temporary.
我是否遗漏了一些允许逗号运算符影响程序语义的东西，或者这是 gcc 中的错误?
Have I missed something that allows the comma operator to affect the semantics of the program, or is this a bug in gcc?
推荐答案
自动移动是基于复制省略的资格:
Automatic move is based on eligibility for copy elision:
§12.8 [class.copy] p32
当满足或将满足省略复制操作的条件时，除了源对象是函数参数的事实，并且要复制的对象由左值指定时，重载决策以选择构造函数首先执行复制，就好像对象由右值指定一样.[...]
When the criteria for elision of a copy operation are met or would be met save for the fact that the source object is a function parameter, and the object to be copied is designated by an lvalue, overload resolution to select the constructor for the copy is first performed as if the object were designated by an rvalue. [...]
当返回表达式是自动对象的名称时，依次允许复制省略.
And copy elision in turn is allowed when the return expressions is the name of an automatic object.
§12.8 [class.copy] p31
在具有类返回类型的函数中的 return 语句中，当表达式是非易失性自动对象的名称时(函数或 catch 除外-clause 参数)与函数返回类型相同的 cv 非限定类型，可以通过将自动对象直接构造到函数的返回值中来省略复制/移动操作
in a
returnstatement in a function with a class return type, when the expression is the name of a non-volatile automatic object (other than a function or catch-clause parameter) with the same cv-unqualified type as the function return type, the copy/move operation can be omitted by constructing the automatic object directly into the function’s return value
插入逗号运算符后，表达式不再是自动对象的名称，而只是对一个对象的引用，从而抑制了复制省略.
With the comma operator inserted, the expression is not the name of an automatic object anymore, but only a reference to one, which suppresses copy elision.
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