Matt Pennington Matt Pennington - 1 month ago 11
C++ Question

c++ std::string.find returns unexected result in boolean expression

I'm trying to search for the presence of '..' in a string that represents a file path on a POSIX system. I'm using std::string.find(".."), and it seems to be finding the correct index, but is not evaluating correctly in boolean expressions.

For example:

#include <string>
#include <stdio.h>


int main( int argc, char *argv[] ) {
std::string a = "abcd";

int apos = a.find( ".." );

bool test1 = a.find( ".." ) >= 0;
bool test2 = apos >= 0;

if ( test1 ) {
printf( "TEST1 FAILED: %ld >= 0!\n", a.find( ".." ) );
}
if ( test2 ) {
printf( "TEST2 FAILED %d >= 0!\n", apos );
}
}


Output:

$ g++ test.cpp -o test
$ ./test
TEST1 FAILED: -1 >= 0!
$


Any ideas why
a.find( ".." )
is not evaluating to -1 in the boolean expressions?

Answer

This has just been asked today.

It's because find returns npos which is an unsigned int, it is initialized using -1 however it's type is unsigned int, so that it is larger than 0.

You should compare find's result with npos not -1.