Do negative numbers return false in C/C++?

Question

When evaluating integers as booleans in C/C++, are negative numbers true or false? Are they always true/false regardless of compilers?

Answer 1

All non-zero values will be converted to true, and zero values to false. With negative numbers being non-zero, they are converted to true.

Quoting from the C++11 standard (emphasis mine):

4.12 Boolean conversions [conv.bool]

1 A prvalue of arithmetic, unscoped enumeration, pointer, or pointer to member type can be converted to a prvalue of type bool. A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true. A prvalue of type std::nullptr_t can be converted to a prvalue of type bool; the resulting value is false.

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Are they always true/false regardless of compilers?

You will only get the above guarantee when your compiler is standards-compliant, or at least, complies with this specific part of the standard. In practice, all compilers have this standard behavior, so there isn't much to worry about.

Answer 2

You can test it yourself by compiling this:

#include <stdio.h>  int main(int argc, char** argv) {     if (-1) {         printf("-1 is true\n");     } else {         printf("-1 is false\n");     }     return 0; } 

Results:

$ gcc -Wall -pedantic test.c -o test-c
$ g++ -Wall -pedantic test.c -o test-cpp
$ ./test-c
-1 is true
$ ./test-cpp
-1 is true

Of course, to answer the second part of your question, "Are they always true/false regardless of compilers?", the only way to be completely sure is to look at the spec. In general though, compilers will warn you if you do something dangerous, and you can see from the output above, that even with "pedantic" warnings, gcc considers this code to be perfectly fine.