Bdtjtk

I was humbly coding away when I ran into a strange situation involving checking the size of a vector. An isolated version of the issue is listed below:

#include <iostream>

#include <string>

#include <vector>



int main() {



  std::vector<std::string> cw = {"org","app","tag"};



  int j = -1;



  int len = cw.size();



  bool a = j>=cw.size();

  bool b = j>=len;





  std::cout<<"cw.size(): "<<cw.size()<<std::endl;

  std::cout<<"len: "<<len<<std::endl;



  std::cout<<a<<std::endl;

  std::cout<<b<<std::endl;



  return 0;

}

Compiling with both g++ and clang++ (with the -std=c++11 flag) and running results in the following output:

cw.size(): 3

len: 3

1

0

why does j >= cw.size() evaluate to true? A little experimenting that any negative value for j results in this weird discrepancy.

The pitfalls here are signed integral conversions that apply when you compare a signed integral value with an unsigned one. In such a case, the signed value will be converted to an unsigned one, and if the value was negative, it will get UINT_MAX - val + 1. So -1 will be converted to a very large number before comparison.

However, when you assign an unsigned value to a signed one, like int len = vec.size(), then the unsigned value will become a signed one, so (unsigned)10 will get (signed)10, for example. And a comparison between two signed ints will not convert any of the both operands and will work as expected.

You can simulate this rather easy:

int main() {

    int j = -1;



    bool a = j >= (unsigned int)10; // signed >= unsigned; will convert j to unsigned int, yielding 4294967295

    bool b = j >= (signed int)10; // signed >= signed; will not convert j



    cout << a << endl << b << endl;



    unsigned int j_unsigned = j;

    cout << "unsigned_j: " << j_unsigned << endl;

}

Output:

1

0

unsigned_j: 4294967295