Why the compiler does not issue a warning when an object std::vector is declared but never used? [duplicate]

Question

#include <vector>

class Object
{
};

int main()
{
    Object myObject;
    std::vector<int> myVector;
}

Compiler emits:

warning: unused variable 'myObject' [-Wunused-variable]

No warning for myVector. Why? Is there any way to enable this?

Answer 1

Whether declaring (and thus initialising and at some point destructung) an arbitrary object has visible side effects cannot be determined in general. The constructor may be calling functions whose definition is not known to the compiler or it may depend on external state or any other aspect which makes the problem undecidable.

In your first case, the constructor is trivial (not even declared), same for the destructor. As Object does not have members, it is clear and easily detectable that Object foo does in fact nothing.

std::vector has a non-trivial constructor which may be allocating memory (external state + function whose definition may not be known (new ...)) together with a non-trivial destructor (also external state + function whose definition may not be known (delete ...)). Reasoning about whether it is safe to remove the declaration (thus emitting a warning hinting that you maybe should) is not possible in this case, thus the compiler has to leave the declaration in the code (and must assume that the declaration is there for a reason).

A prime example is std::lock_guard which is used to lock a mutex when it is constructed and unlock it automatically when it is destructed. The mutex is thus held as long as the object is in scope; generally you would not access the std::lock_guard object at all though—nevertheless it is useful to have it declared. This is the RAII principle at work.

Emitting a warning in such cases would be a nuisance, leading to people turning off the warning, which in turn would render the warning useless. (The compiler may even be designed in such a way that it only emits the warning if it has removed the declaration during optimisation, which is also the reason why some warnings only show up if certain optimisations are enabled.)