此语句：

Foo(x);

不是函数调用，也不是构造函数调用。这只是一个声明，说 x 是 Foo 类型。括号在声明符 x 周围是可选的，因此等效于：

is not a function call, or a constructor call. It's just a declaration, that says x is of type Foo. The parentheses are optional around the declarator x, so it's equivalent to:

Foo x;

这当然会产生错误，因为您已经有 std :: string 名为 x ，并且您不能为同一范围内的多个实体提供相同的名称。

This of course gives an error, since you already have a std::string named x, and you can't give the same name to multiple entities in the same scope.

请注意表达式：

Foo(x)

与上面的声明（带有; ）不同。根据使用的上下文，该表达式可能表示不同的含义。

is different than the statement above (with the ;). This expression can mean different things depending on the context it is used in.

例如，这段代码：

Foo foo = Foo(x);

非常好。这确实从表达式 Foo（x）复制一个名为 foo 的变量的初始化，这是一个临时的 Foo 由参数 x 构造。 （这里不是特别重要，但是从c ++ 17开始，在右侧没有临时对象；该对象只是在适当的位置构造）。

is perfectly fine. This does copy initialization of a variable named foo, from the expression Foo(x), which is a temporary Foo constructed from the argument x. (It's not particularly important here, but from c++17, there's no temporary on the right hand side; the object just gets constructed in place).