C/C++: warning: control reaches end of non-void function [-Wreturn-type]

Meaning

Flowing off the end of a non-void function is undefined behaviour in C++ (the caller may use garbage, or execution may fall into the next function). It often shows up as a crash only at -O2. Return on every path, or restructure with a final return / throw.

Common causes

• An if/else if chain without a final else return
• A switch without a default that returns
• Return placed only inside a loop that might not run
• Function body missing return after a catch block

Detailed fix by platform

Code examples

int sign(int x) {
    if (x > 0) return 1;
    if (x < 0) return -1;
    // warning: control reaches end of non-void function (x == 0)
    return 0;                  // fix
}

How to diagnose

1. Paths — Which input reaches the end?
2. Switch — Is there a default?
3. Flags — Compiled with -Wall?

More related errors