Search code examples
c++stringbooleanposix

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?

Solution

  • 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.