An Unfortunate Assignment

This program may not behave as you expect.

#include <iostream> using namespace std; int main() { int n; /* Read and test a value. */ cin >> n; if(n = 5) cout << "You entered five!" << endl; else cout << "You FAILED to enter five!" << endl; cout << "Your number: " << n << endl; }

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Many compilers will issue a warning for this code, though sometimes the message is a difficult to decipher. (In C++, it's usually a good idea to find the cause of all compiler warning messages.) This code is legal C++ because of several language rules, most of which we've mentioned before:

Assignments are expressions. An assignment's type and value are those of the variable assigned.
As noted, int converts automatically to bool under rules described earlier, so an integer expression may be used as a test expression.
Therefore n = 5 is a perfectly legal if test which is always true.

One other odd consequence of the conversion rule might be of interest to any of you who has experience with the wonderful Python languge. In Python, an expression like 7 > 6 > 5 is legal and evaluates to true, which is quite nice. In Java, 7 > 6 > 5 is not legal, which is quite reasonable. In C++, 7 > 6 > 5 is legal and evaluates to false, which is quite annoying. I'll go into the reasons if you ask, but I'm not so worried about you stumbling onto this one by accident.