When do compilers inline C++ code?

Question

In C++, do methods only get inlined if they are explicitly declared inline (or defined in a header file), or are compilers allowed to inline methods as they see

Answer 1

Some of the situations where inline expansion may NOT work are:

1. For functions returning values, if a loop, a switch, or a goto exists
2. For function not returning values, if a return statement exits;
3. If functions contain static variables
4. If inline functions are recursive.

Inline expansion makes a program run faster because the overhead of a function call and return statement is eliminated. However, it makes the program to take up more memory because the statements that define the inline functions are reproduced at each point where the function is called. So, a trade-off becomes necessary.

(As given in one of my OOP books)

Answer 2

Your compiler's documentation should tell you since it is implementation dependent. For example, GCC according to its manual never inlines any code unless optimisation is applied.

If the compiler does not inline the code, the inline keyword will have the same effect as static, and each compilation unit that calls the code will have its own copy. A smart linker may reduce these to a single copy.

Answer 3

Compilers may ignore your inline declaration. It is basically used by the compiler as a hint in order decide whether or not to do so. Compilers are not obligated to inline something that is marked inline, or to not inline something that isn't. Basically you're at the mercy of your compiler and the optimization level you choose.

Answer 4

Yes, the compiler can inline code even if it's not explicitly declared as inline.

Basically, as long as the semantics are not changed, the compiler can virtually do anything it wants to the generated code. The standard does not force anything special on the generated code.

Answer 5

If I'm not mistaken, when optimizations are turned on, the compiler will inline any suitable routine or method.