临时std ::字符串返回垃圾 [英] Temporary std::strings returning junk

问题描述

我还在学习c ++，所以请和我一起玩。我正在写一个简单的包装程序boost文件系统路径 - 我有返回临时字符串的奇怪问题。这是我的简单类（这不是精确，但很接近）：

I'm still learning c++, so please bear with me. I'm writing a simple wrapper around boost filesystem paths -- I'm having strange issues with returning temporary strings. Here's my simple class (this is not exact, but pretty close):

typedef const char* CString;
typedef std::string String;
typedef boost::filesystem::path Path;

class FileReference {

    public:

        FileReference(const char* path) : mPath(path) {};

        // returns a path
        String  path() const {
            return mPath.string();
        };

        // returns a path a c string
        CString c_str() const {
            return mPath.string().c_str();
        };

    private:

        Path    mPath;

}

使用下面的小测试代码：

With the little test code below:

FileReference file("c:\\test.txt");

OutputDebugString(file.path().c_str()); // returns correctly c:\test.txt
OutputDebugString(file.c_str());        // returns junk (ie îþîþîþîþîþîþîþîþîþîþî.....)

处理临时，但我不知道为什么会是 - 一切都应该正确复制？

I'm pretty sure this has to deal with temporaries, but I can't figure out why that would be -- shouldn't everything be copying correctly?

推荐答案

看起来像 mPath.string（）按值返回一个字符串。一旦FileReference :: c_str（）返回，临时字符串对象就被销毁，所以它的c_str（）变得无效。使用这样的模型，不能创建 c_str（）函数，而不为字符串引入某种类或静态级变量。

Looks like mPath.string() returns a string by value. That temporary string object is destructed as soon as FileReference::c_str() returns, so its c_str() becomes invalid. With such a model, it's not possible to create a c_str() function without introducing some kind of class- or static-level variable for the string.

考虑以下替代方法：

//Returns a string by value (not a pointer!)
//Don't call it c_str() - that'd be misleading
String str() const
{             
     return mPath.string();
} 

或

void str(String &s) const
{             
     s = mPath.string();
} 

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