Show Warning for "anonymous" variable creation in c++

Question

I've a class like this:

class ScopedLock {
public:
    ScopedLock (Locker *lock) { /*...*/ }
    ~ScopedLock () { /*...*/ }
};

Normally it's called like (this will call the constructor/destructor of ScopedLock at the correct place) :

{
    ScopedLock l(&locker);
    // ...
}

I've accidentally called it like:

{
    ScopedLock(&locker);
    // ...
}

What is the name for such a "thing"? Unused anonymous local variable?

Is there a possibility to prevent this? Is there a compiler warning available for such a "thing"?

You 've created an _unnamed temporary_ of type `ScopedLock` that lives briefly and then dies at the `;`. — Ron, Jul 12 '18 at 10:52
_"Normally it's called like: ..."_ So you intended a function forward declaration there? Be aware of the _most vexing parse_! — πάντα ῥεῖ, Jul 12 '18 at 10:54
Also I don't think there should be a warning generated, there might be cases, where such is done intentionally. — πάντα ῥεῖ, Jul 12 '18 at 10:57
I would have expected `[[nodiscard]] ScopedLock (...) { ...}` to issue the wanted warning, but instead, I got a warning that it cannot apply the attribute to constructor :-/ — Jarod42, Jul 12 '18 at 10:57
Amazing video about this here: http://www.youtube.com/watch?v=lkgszkPnV8g&t=29m40s — Fantastic Mr Fox, Jul 12 '18 at 11:16
Maybe [this question](https://stackoverflow.com/questions/47602539/emulate-c-sharp-lock-statement-in-c) is also interesting in this context. — Jodocus, Jul 12 '18 at 12:15

Ron · Answer 1 · 2018-07-12T11:54:04.253

You have created an unnamed temporary / nameless temporary object of type ScopedLock that lives briefly and then dies at the ;. The compiler does not issue a warning as it assumes you are doing something useful with it. It does not go into a ctor body to inspect if that might be the case. For example you want to create a temporary where your constructor might do some work:

ScopedLock() {
    // do some work
}

You can't force a compiler to show a warning for such use-cases and there is no such flag in GCC.

The following SO posts can prove beneficial:

user7860670 · Answer 2 · 2018-07-12T11:21:30.510

You can make a construction function marked as nodiscard that should be invoked instead of constructor.

#include <iostream>

class ScopedLock final
{
    private: ScopedLock(int, int) {std::cout << "constructed" << std::endl;}
    private: ScopedLock(void)                            = delete;
    private: ScopedLock(ScopedLock const &)              = delete;
    private: ScopedLock(ScopedLock &&)                   = delete;
    private: ScopedLock & operator =(ScopedLock const &) = delete;
    private: ScopedLock & operator =(ScopedLock &&)      = delete;

    public: [[nodiscard]] static ScopedLock
    construct(int x, int y)
    {
        return ScopedLock{x, y};   
    }
};

int main()
{
    ScopedLock(12, 123); // Error
    ScopedLock::construct(12, 123); // Warning
    auto lock(ScopedLock::construct(12, 123)); // Ok
}

online compiler

Show Warning for "anonymous" variable creation in c++

2 Answers2