C++ Primer, Fifth Edition

Beware of Confusing Equality and Assignment Operators

The fact that we can use assignment in a condition can have surprising effects:

if (i = j)

The condition in this if assigns the value of j to i and then tests the result of the assignment. If j is nonzero, the condition will be true. The author of this code almost surely intended to test whether i and j have the same value:

if (i == j)

Bugs of this sort are notoriously difficult to find. Some, but not all, compilers are kind enough to warn about code such as this example.