使用匿名Stringstream构造一个字符串 [英] Use an Anonymous Stringstream to Construct a String

问题描述

我想直接读入一个字符串，如下所示：

I want to read directly into a string with code like this:

std::string myString(( std::ostringstream() << myInt << " "
                                            << myFloat << " "
                                            << std::boolalpha << myBool ).str());

但VS2012让我抱怨 basic_ostream 没有 str（）方法。

But VS2012 is giving me a complaint that basic_ostream doesn't have an str() method.

有一种方法可以用匿名stringstream ？运算符<< 对于流返回<$ c>

Is there a way to do this with an anonymous stringstream?

推荐答案

$ c> std :: ostream& ，它没有 str（）成员函数。您需要使用cast。

The operator<< for streams returns std::ostream & which doesn't have str() member function. You need to use cast.

static_cast<std::ostringstream&>(std::ostringstream() << etc).str()

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+03和C ++ 11使用临时流！

C ++ 03

在C ++ 03中） std :: ostringstream（）创建一个临时对象，这意味着<$ c $的非成员重载 不能为第一个 调用 运算符，因为它们都接受第一个参数 std :: ostream& ，它不能绑定到临时对象。临时对象将能够调用只有成员函数。

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Differences between C++03 and C++11 when using temporary stream!

C++03

But be aware (in C++03) that std::ostringstream() creates a temporary object, which means the non-member overloads of operator<< cannot be invoked for the first << because all of them accepts the first argument as std::ostream& which cannot bind to the temporary object. The temporary object will be able to invoke only member functions.

这意味着，而不是 string ：

static_cast<std::ostringstream&>(std::ostringstream() << "XYZ").str()

$ c> char const * 作为参数是一个非成员函数，不能被调用，所以上面的代码最后调用成员函数 void const * 作为参数，因此XYZ隐式转换为 void const * ，它打印字符串文字的地址。

Because the overload which takes char const* as argument is a non-member function, which cannnot be invoked, so the above code end up calling the member function which takes void const* as argument and thus "XYZ" is implicitly converted into void const*, which prints address of string literal.

一旦临时调用成员函数，剩余的链接 ; 可调用非成员重载，因为成员函数返回 std :: ostream& 其中现在可以绑定到运算符的非成员重载的第一个参数 。因此，以下代码将打印地址（而不是XYZ），后跟字符串ABC：

Once the temporary invokes the member function, the remaining chained << may invoke non-member overloads, because the member function return std::ostream& which now can bind to the first argument to the non-member overloads of operator<<. So this following code would print an address (instead of "XYZ") followed by string "ABC":

static_cast<std::ostringstream&>(std::ostringstream() << "XYZ" << "ABC").str()

在线演示：

• GCC输出 without c> -std = c ++ 11


• Clang输出 无 -std = c ++ 11

• GCC output without -std=c++11
• Clang output without-std=c++11

在C ++ 11中，此问题已通过添加非成员函数（27.7.3.9），它将第一个参数作为右值引用，然后将该调用转发到适当的函数（成员或非成员）。因此，它打印 XYZ 后跟 ABC ：

In C++11, this issue has been fixed by adding a non-member function (27.7.3.9) which takes the first argument as rvalue reference which then forwards the call to the appropriate function, either member or non-member. So it prints XYZ followed by ABC:

• GCC输出 使用 -std = c ++ 11


• 剪辑输出 包含 -std = c ++ 11

• GCC output with-std=c++11
• Clang output with -std=c++11

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