使用std :: string而不是char * bad？

Question

I'm beginning with C++ and I am little confused whether to use std::string or char* .

Lets say I need to concatenate a few strings. I'd do it like this - I think it's the shortest way.

std::string data = "some data";
const char *result = std::string("start" + data + "end" + "something else").c_str();

Question is: Should I do it this way or use c-functions like strcpy() , strcat() etc.? Is using std::string slower, less effective than char* ?

Thanks

Answer 1

const char *result = std::string("start" + data + "end" + "something else").c_str(); is a recipe for disaster .

The right hand of the assignment is using an anonymous temporary and will leave a dangling pointer result .

The idea of converting std::string data = "some data"; to char* data = "some data"; is not emphasising the fact that "some data", after the conversion from std::string becomes read-only memory . Modification of the memory to which data is pointing to is undefined behaviour. At least use a const char* so compilation will fail on such an attempt.

All these problems go away if you use std::string . There's your answer. It considerably simplifies things.

Answer 2

As many have already said in the comments, always use std::string . It is better than null-terminated strings (C / char* strings) in almost every way.

However the way you used string in your example is not correct and involves undefined behaviour.

const char *result = std::string("start" + data + "end" + "something else").c_str();

In this line you created a temporary string . As I hope you know, temporary variables only last until the end of the row (as far as this example goes at least). So after this row the string will not exist anymore. A string has an internal array on heap in which it stores its string . The function string::c_str() returns a pointer to the first character of that array. But because the string object gets destroyed at the end of the line, the pointer result will point to a deleted part of the memory and accessing it is undefined behaviour.

std::string data = "some data";
std::string bigData("start" + data + "end" + "something else");
const char *result = bigData.c_str();

If you wrote your code like this, then there is no undefined behaviour since result 's lifetime is shorter than the string bigData .