Thread: Four questions on std::stringstream

Greetings everyone! I have a few hopefully pretty easy questions on std::stringstream

1) How do you "reset" a stringstream so that it can be used again? I looked around on Google and found ss.str(""); (where ss is an object of type std::stringstream) but this seems like an ugly hack and I was wondering if there was a "cleaner" way to do it.

2) Is it possible to get a C style string from a stringstream? I tried ss.str().c_str() but I don't think that works.

3) How can you get the length of a stringstream? Does ss.str().length() work?

4) In the Visual Studio debugger, how can I display the contents of a stringstream? the Watch window can't seem to parse ss.str().c_str().

Thanks for the help.

1) str("") is the only way I know of
2) that's what I usually use
3) yes works
4) What is a Visual debugger
Kurt

Did you try testing 1 through 3? It's the best way to test things. What did it do in the testing?

Originally Posted by CornedBee

2) ss.str().c_str() will work, but only as long as the temporary std::string stays alive.

This is true, but the constraint is more general than that. The string itself needs to remain unmodified. So

Code:

     std::string some_string(some_content);
     const char *p = some_string.c_str();
     
     //   some operation that modifies the content of some_string()

    strcpy(some_buffer, p);    // undefined behaviour as p is effectively a dangling reference

ss.str() yields a temporary string with limited lifetime, and ss.str().c_str() is only valid as long as that temporary remains alive. Being destructed, so it is no longer alive, is a rather significant modification of that temporary string.

Originally Posted by CornedBee

Which is long enough to pass the c-string to a function (like strcpy), but not more than that.

The other obvious caveat is that the c_str() method returns a const pointer, which is a fairly broad hint you're not supposed to modify the data it points at. So

Code:

    char x[10];
    strcpy(x, ss.str.c_str());

is OK (as long as the length of the string with terminating zero does not exceed 9) but this;

Code:

    char x[] = "Hello";
    strcpy(ss.str.c_str(), x);

will not work; the compiler will complain bitterly as the first argument of strcpy() is required to be non-const.