string and const char* and .c_str()?

Question

I'm getting a weird problem and I want to know why it behaves like that. I have a class in which there is a member function that returns std::string. My goal to convert this string to const char*, so I did the following

    const char* c;
    c = robot.pose_Str().c_str();  // is this safe??????
    udp_slave.sendData(c);

The problem is I'm getting a weird character in Master side. However, if I do the following

    const char* c;
    std::string data(robot.pose_Str());
    c = data.c_str();
    udp_slave.sendData(c);

I'm getting what I'm expecting. My question is what is the difference between the two aforementioned methods?

Answer 1

It's a matter of pointing to a temporary. If you return by value but don't store the string, it disappears by the next sequence point (the semicolon).

If you store it in a variable, then the pointer is pointing to something that actually exists for the duration of your udp send

Consider the following:

int f() { return 2; }


int*p = &f();

Now that seems silly on its face, doesn't it? You are pointing at a value that is being copied back from f. You have no idea how long it's going to live.

Your string is the same way.

Answer 2

.c_str() returns the the address of the char const* by value, which means it gets a copy of the pointer. But after that, the actual character array that it points to is destroyed. That is why you get garbage. In the latter case you are creating a new string with that character array by copying the characters from actual location. In this case although the actual character array is destroyed, the copy remains in the string object.