C ++返回临时对象的混乱 [英] C++ returning temporary objects confusion

问题描述

我有一个相当基本的C ++问题，考虑一个函数，该函数需要一些输入参数，并根据如下参数创建一个std::string:

std::string constructString( int some_parameter ) {

    std::stringstream ss;

    // Construct a string (arbitrarily complex)
    ss << "Some parameter is " << some_parameter << " right now";

    return ss.str();    //Am I not returning a temporary object here?
}

我知道当函数返回时，stringstream-object将超出范围，但这也不会使构造的字符串无效吗?

如果我将返回类型更改为const char *却返回了ss.str().c_str()会发生什么?

上面的代码似乎可以正常工作，但是我怀疑那是因为当我使用它时，包含临时"对象的内存还没有被其他东西覆盖吗?

我必须承认，在这样的情况下，我通常会感到困惑，如果有人可以向我解释整个临时对象"(或者只是将我指出正确的方向)，我将不胜感激./p>

提前谢谢

解决方案

您将返回一个临时对象，但是由于按值返回它，因此将创建副本.如果返回指针或对临时对象的引用，那将是一个错误.

如果将返回类型更改为const char *并返回ss.str().c_str()，您将返回指向ss.str()返回的临时std::string缓冲区的指针，那将是不好的.

I've got a rather basic C++ question, consider a function that takes some input parameters and creates a std::string from those parameters like the one below:

std::string constructString( int some_parameter ) {

    std::stringstream ss;

    // Construct a string (arbitrarily complex)
    ss << "Some parameter is " << some_parameter << " right now";

    return ss.str();    //Am I not returning a temporary object here?
}

I understand that the stringstream-object will go out of scope when the function returns, but doesn't that invalidate the constructed string as well?

What would happen if I changed the return type to const char * and returned ss.str().c_str() instead?

Code like the above seems to work, but I suspect that's just because the memory containing the 'temporary' object has not yet been overwritten with something else when I use it?

I have to admit, I'm rather confused in such situations in general, I'd appreciate it if someone could explain this whole "temporary objects"-thing to me (or just point me in the right direction).

thx in advance

解决方案

You are returning a temporary object, but because you return it by value, the copy is created. If you return pointer or reference to temporary object, that would be a mistake.

If you change the return type to const char * and return ss.str().c_str() you would return pointer to some buffer of temporary std::string returned by ss.str() and that would be bad.

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