What Happens When a Function with a Return Type Lacks a Return Statement?

Function without Return Value: Undefined Behavior and Runtime Consequences

When functions with return types are missing a return statement, the compiler raises a warning, as exemplified in the code snippet provided. However, the behavior at runtime in such situations is not straightforward.

According to the ISO C standard (section 6.6.3), leaving a non-void function without a return statement constitutes undefined behavior. This means that the actual return value in such cases is unpredictable and cannot be relied upon.

In the code provided, the function doSomethingWith lacks a return statement when condition is false. As a result, the behavior at runtime is undefined. However, it appears that the function returns a value of 0, which is not the intended behavior.

It's important to note that the return value is only undefined for POD (Plain Old Data) datatypes, such as int, float, and double. For non-POD datatypes, like classes and structs, a default constructor is invoked to construct the return value, which may not have the intended behavior.

Implications

The undefined behavior in such situations can lead to unpredictable results or even program crashes. Therefore, it's crucial to ensure all paths in a function with a return type have a valid return value.

Recommendations

To prevent undefined behavior, it's recommended to:

• Explicitly return a value from all paths within the function.
• Use return statements with expressions that consistently return the desired result.
• Avoid leaving a function without a return statement unless it's a void function.

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