Strange String problem

[Chris Walton]

I've been programming in C++ for about a year now, and I've run into a strange issue. I've found a workaround for the issue, but I want to understand what caused the issue in the first place.

I am using a string to pass a value to a function that requires a char*. So, the function call looks something like this:

int i = 2;
string foo = "foo bar baz " + i;
function(stuff,stuff,stuff,(char*)foo.c_str(),stuff,stuff);

Don't worry about what "stuff" is, that is all functional.

Now, when the function displays my string, it is missing the first 4 characters. It's easy enough to work around, I just placed 4 spaces before "foo". Obviously, this is not ideal.
I am still not a master of pointers, so I was wondering if that might be the issue? Or what are some other reasons this could happen?

[ResidentBiscuit]

Can you not just pass it as foo.c_str()?

[Chris Walton]

I tried to do that at first, but it gives me the error: "argument of type const char* is incompatible with parameter of type char*"

I'm using Visual Studio 2010 Express as my IDE if that is of any help.

[vlad from moscow]

It is not a strange issue as you think. Let consider the expression

"foo bar baz " + i

The string literal is implicitly converted to the pointer to its first element. It looks like

const char *p = "foo bar baz ";

then i is added that is you get as the result the expression p + i. If i = 2 then p + i will point to the second 'o' in the word "foo".

I only wonder why you are saying about 4 missing characters then there can be only two first missing characters due to the pointer arithmetic.

[ResidentBiscuit]

Is there any reason why you have to use char* ? The only ways around this I can think of are pretty messy.

EDIT:
And vlad comes in with the explanation! I didn't even look at your "..." + i deal.

[Chris Walton]

Actually, in my real code, i is 4. I didn't think that was relevant, so I didn't include that. Glad to have an explanation that makes sense, thank you vlad!

I do need to have the integer in there, so how should I work around this? I'm trying to see if I can do something with foo.append(""+i); with no success.

[ResidentBiscuit]

I do everything like that using stringstreams. Can throw whatever you want in there and have it give you a nice string. I believe this is the standard C++ way of doing things.

[Chris Walton]

stringstreams are always handy. I'm actually in the process of re-writing a 2000 line X-Plane plugin, and stringstreams are what I used in the original code. I'm trying to optimize the new code, so that's why I asked. But, if stringstreams are the standard, then I'll use them again.

Thank you for your help guys!

[vlad from moscow]

A simple way is

string foo = "foo bar baz " + std::to_string( i );

Only you should take into account that MS VC++ 2010 does not contain the overload function std::to_string for the argument of type int. So you could use std::to_string( static_cast<long long>( i ) ) if you are using MS VC++ 2010

[Chris Walton]

Once again, thank you vlad. As I said, I'm trying to optimize the new code, and it would seem to me that the overhead from the to_string function is probably less than that of a stringstream, so I'm going with that.

[ResidentBiscuit]

Don't quote me on this, but I think the performance of stringstream is pretty solid. I used it for a time-sensitive project awhile back and did it well. Faster than a manual conversion

EDIT:
After some research, it appears my results were the minority.