const引用默认值 [英] const reference default-value

问题描述

可能重复：

如何初始化默认值为的类的函数参数

#include <string>

void foo1(const std::string& s = std::string());

void foo2(std::string& s = std::string());

void foo3(const std::string s = std::string());

void foo4(std::string s = std::string());

错误foo2（）：'std :: string& s'有类型'std :: string {aka std :: basic_string< char>}'

我理解编译器的观点，不会得到这不适用于 foo1（）以及。

I understand the compiler's point, but I don't get how this does not apply to foo1() as well.

推荐答案

你不能像foo2那样对一个临时对象进行非const引用。

You can't take a non-const reference to a temporary like foo2 does.

注意，这不是默认的参数。对于函数变量，您会遇到相同的错误： http://ideone.com/g7Tf7L

Notice that this isn't specifically default parameters. You get the same error for function variables: http://ideone.com/g7Tf7L

#include <string>
using std::string;

#include <iostream>
using std::cout; using std::endl;

int main()
{
    string s1        = string("s1"); // OK, copy it
    const string& s2 = string("s2"); // OK, const reference to it
    string& s3       = string("s3"); // ERROR! non-const reference not allowed!

    cout
            << s1 << ", "
            << s2 << ", "
            << s3 << endl;
    return 0;
}

当你对一个临时变量使用const引用时，扩展到引用的生命周期（§12.2，引自我的C ++ 11草案n3337的副本）：

When you take a const reference to a temporary, the lifetime of the temporary is extended to the lifetime of the reference (§12.2, quoted from my copy of C++11 draft n3337):

有两个上下文其中在与富勒氏结节的不同点处破坏临时。

There are two contexts in which temporaries are destroyed at a different point than the end of the fullexpression.

...

第二个上下文是当引用绑定到临时时。引用绑定到的临时对象或临时对象（引用绑定到的子对象的完整对象）在引用的生命周期内仍然存在，除了：

The second context is when a reference is bound to a temporary. The temporary to which the reference is bound or the temporary that is the complete object of a subobject to which the reference is bound persists for the lifetime of the reference except:

• 在构造函数的ctor-initializer（12.6.2）中临时绑定到引用成员，直到构造函数退出。


• 在函数调用（5.2.2）中，引用参数的临时绑定将持续到包含调用的完整表达式完成为止。


• 在函数return语句（6.6.3）中绑定到返回值的临时的生命周期不会被延长;临时将在return语句中的full-expression结尾处销毁。


• 暂时绑定到新初始化程序（5.3.4）中的引用，直到完成包含新初始化程序的完整表达式。

• A temporary bound to a reference member in a constructor’s ctor-initializer (12.6.2) persists until the constructor exits.
• A temporary bound to a reference parameter in a function call (5.2.2) persists until the completion of the full-expression containing the call.
• The lifetime of a temporary bound to the returned value in a function return statement (6.6.3) is not extended; the temporary is destroyed at the end of the full-expression in the return statement.
• A temporary bound to a reference in a new-initializer (5.3.4) persists until the completion of the full-expression containing the new-initializer.

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