初始化:括号与等号

initialization: parenthesis vs. equals sign(初始化:括号与等号)

问题描述

有什么区别

T a(b);

和

T a = b;

和

T a = T(b);

?

推荐答案

T a( b );

是直接初始化，除非它被解析为函数声明，在这种情况下它是一个函数声明.

is direct initialization, unless it parses as a function declaration, in which case it's a function declaration.

T a = b;

是复制初始化，这意味着它就像在右侧构造一个临时对象一样工作，然后a是复制构造的，或者在C中++11 及更高版本，可能从该临时移动构造.

is copy initialization, which means that it works as if a temporary object is constructed on the right hand side, and that a is then copy constructed or, in C++11 and later, possibly move constructed, from that temporary.

编译器可以在可能的情况下自由地删除(删除)临时+复制/移动，但是复制或移动构造函数，无论在逻辑上使用哪个，都必须仍然是可访问的，而不是显式.

The compiler is free to elide (remove) the temporary+copying/moving whenever it can, but a copy or move constructor, whichever would be logically used, must still be accessible and not explicit.

例如，在 C++03 中，您不能复制初始化 std::ostringstream，因为它没有复制构造函数.在 C++11 中，如果初始化程序是临时的，则可以复制初始化 ostringstream，这会导致逻辑移动构造(但通常会被省略，优化掉).比如这个拷贝初始化声明，

For example, in C++03 you cannot copy-initialize a std::ostringstream, because it doesn't have a copy constructor. In C++11 you can copy-initialize an ostringstream if the initializer is a temporary, which then results in a logical move construction (which however will usually be elided, optimized away). For example, this copy initialization declaration,

ostringstream s = ostringstream( "blah" );

…不编译为 C++03，因为在 C++03 中，复制初始化调用了类的复制构造函数，而该构造函数不存在.然而，它确实编译为 C++11，因为在 C++11 中，复制初始化调用了移动构造函数.虽然(为了保持其作为流的错觉)std::ostringstream 不能直接复制，但它可以移动.

… doesn't compile as C++03, because in C++03 the copy initialization invokes the class' copy constructor, which doesn't exist. It does however compile as C++11, because in C++11 the copy initialization invokes the move constructor. And while (to maintain its illusion of being a stream) a std::ostringstream can't be directly copied, it can be moved.

另一个这样的区别:在 C++03 中，只有复制初始化语法支持 花括号 初始化器，在 C++03 中，当 T 是聚合时可以使用它类型，例如原始数组.在 C++11 中，花括号符号已被扩展并概括为 统一初始化语法，因此它也可以用于直接初始化.所以下面直接初始化声明，

Another such difference: in C++03 only the copy initialization syntax supports curly braces initializer, which in C++03 you can use when T is an aggregate type such as a raw array. In C++11 the curly braces notation has been extended and generalized as a uniform initialization syntax, so it can be used also with direct initialization. And so the following direct initialization declaration,

int v[]{ 3, 1, 4, 1, 5, 9, 2, 6, 5, 4 };

…不编译为 C++03，但编译为 C++11 及更高版本.

… does not compile as C++03, but does compile as C++11 and later.

= 复制初始化语法是 C 的原始初始化语法.

The = copy initialization syntax is the original initialization syntax from C.

在 C++11 及更高版本中，由于移动语义，它可以用于比 C++03 更广泛的情况，例如与 std::ostringstream.

And in C++11 and later, due to move semantics, it can be used in a much wider range of cases than in C++03, such as with a std::ostringstream.

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