Using C++ strings with C library functions

Question

The error occurs when I try to use the atoi(const char*) function in the following line...

externalEncryptionRawHolder[u] = atoi(parser.next());

The 'parser' object is a string parser and the 'next' method returns a string. I think the error has something to do with the fact that the string within the 'atoi' function isn't a constant... but I'm not sure. The gist of the error is 'cannot convert string to const char *'. How can I make my string constant? Any help would be very appreciated (by the way, in case you're wondering what the index 'u' is, this is within a 'for' loop).

Seth Carnegie · Accepted Answer · 2012-02-04 22:40:07Z

7

You have to call c_str() on the string object to get a const char*:

externalEncryptionRawHolder[u] = atoi(parser.next().c_str());

Note, though, that you should not do this:

const char* c = parser.next().c_str();

Because c will point to the memory that was managed by the string returned by parser.next(), which gets destroyed at the end of the expression, so then c points to deallocated memory. The first example is ok though because the string is not destroyed until after atoi has returned.

edited Feb 4, 2012 at 22:40

answered Feb 4, 2012 at 22:32

Seth Carnegie

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Comments

Drew Dormann · Accepted Answer · 2012-02-04 23:40:13Z

1

string::c_str() will convert a string to a const char*, which is what atoi expects.

externalEncryptionRawHolder[u] = atoi(parser.next().c_str());

edited Feb 4, 2012 at 23:40

answered Feb 4, 2012 at 22:33

Drew Dormann

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3 Comments

Monkeyanator Over a year ago

Thanks so much! I'll thank you as soon as able (7 minutes). Say, why does that work?

Seth Carnegie Over a year ago

@Monkeyanator just because next() returns a string which is incompatible with what the function atoi expects, which is a const char*. string has a member function c_str() though, which returns a const char* to the string it manages internally, which atoi can work with

Drew Dormann Over a year ago

@Monkeyanator: Seth is right. string and char* can both represent text, but they aren't the exact same thing. This works because you're converting one form to the other.

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