c_str()返回空字符串 [英] c_str() returns empty string

问题描述

由于某种原因c_str()返回空字符串，参数 const chart ** out_function 将保存诸如 fopen 之类的文件操作的方法名称，所以基本上我要做的是将字符串转换为c_str()，但是下面得到的是一个空字符串

For some reason c_str() returns empty string, the parameter const chart**out_function will hold a method name for file operations like fopen so basically what I do is converting a string I have to c_str() but I get an empty string below is how I do the calls

在这一部分中，我只是准备一个带有操作名称的字典，您会注意到我只是以字符串形式发送"fopen"

In this part I just prepare a dictionary with an operation name, as you can notice I am just sending "fopen" as string

    pp::VarDictionary fileOp;
    pp::VarArray args;
    args.Set(0, "filename.txt");
    args.Set(1, "wb");
    fileOp.Set("args", args);
    fileOp.Set("cmd", "fopen");

此函数将解析上面发送的字典，并在 out_params

This function will parse the dictionary sent above and return the name of the function in out_function and args in out_params

    int ParseMessage(pp::Var message, const char** out_function,
        pp::Var* out_params) {

我使用此行代码将字符串转换为c_string，但它返回空文本

I use this line of code to convert the string to c_string, but It returns empty text

*out_function =  cmd_value.AsString().c_str();

这是完整的代码，它基于Google Native Client，但同时又是标准的C/C ++代码

here is the full code, it is based on Google Native Client but at the same time it is standard C/C++ code

http://pastebin.com/S4P8aZqL

推荐答案

c_str()的结果仅在 std :: string 对象符合以下条件时有效产生的结果是有效的.

The result of c_str() is only valid as long as the std::string object that produced that result is valid.

在您的情况下， AsString()调用会生成一个临时 std :: string 对象，然后该对象立即销毁.之后，该 c_str()调用的结果不再有意义.尝试访问该指针所指向的内存会导致未定义的行为.

In your case, AsString() call produces a temporary std::string object which is then immediately destroyed. After that the result of that c_str() call no longer makes any sense. Trying to access the memory pointed by that pointer leads to undefined behavior.

不要尝试存储由 c_str()返回的指针.如果长时间需要该字符串作为C字符串，请自己为其分配内存缓冲区，然后将 c_str()的结果复制到该缓冲区.

Don't attempt to store the pointer returned by c_str(). If you need that string as a C-string for an extended period of time, allocate memory buffer for it yourself and copy the result of c_str() to that buffer.

另一个更好的想法是不要急于转换为C字符串.返回结果为 std :: string ，并在最后一刻调用它: c_str():当您真正需要C字符串时.

Another (much better) idea would be not to rush the conversion to C-string. Return the result as std::string and call c_str() on it at the very last moment: when you reall really really need a C-string.

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