Welcome to OStack Knowledge Sharing Community for programmer and developer-Open, Learning and Share
Welcome To Ask or Share your Answers For Others

Categories

0 votes
400 views
in Technique[技术] by (71.8m points)

c++ - string and const char* and .c_str()?

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?

See Question&Answers more detail:os

与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
Welcome To Ask or Share your Answers For Others

1 Answer

0 votes
by (71.8m points)

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.


与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…
Welcome to OStack Knowledge Sharing Community for programmer and developer-Open, Learning and Share
Click Here to Ask a Question

...