RAII using custom deleter of unique_ptr

Question

I want to do automatic cleanup using std::unique_ptr but to initialize the unique_ptr I need an address to point to. How can I avoid the noUse variable?

bool noUse;
auto deleter = [](bool *){ DESTROY_SOMETHING }; 
std::unique_ptr<bool, decltype(deleter)> upp(&noUse, deleter); // I want to avoid the usage of this extra noUse variable
START_SOMETHING

//COMPLEX IF ELSE RETURN LOGIC

**UPDATE: ** I finally resorted to this code. Thanks everyone for quick responses.

auto deleter = [](void *){ DESTROY_SOMETHING };
std::unique_ptr<void, decltype(deleter)> upp(nullptr, deleter);

if(enabled)
{
   //START_SOMETHING
   upp.reset(reinterpret_cast<void *>&upp);
}

//COMPLEX IF ELSE RETURN LOGIC

Answer 1

As long as the custom deleter doesn't require noUse to have any specific value, you can use some arbitrary integer literal (other than zero, that would make it a null pointer):

auto deleter = [](bool *){ DESTROY_SOMETHING };

// slightly longer but more robust:          reinterpret_cast<bool*>(&upp)
std::unique_ptr<bool, decltype(deleter)> upp(reinterpret_cast<bool*>(1), deleter);
// ...

This is a bit of a hack, and there are better solutions, such as std::experimental::scope_exit. It also doesn't make much sense that this is using a bool*, why not just a void* if we don't care about the type?

You can also build your own solution, see:

• Simple way to execute code at the end of function scope
• Call a function before function exits

@Joe has written a solution very similar to std::experimental::scope_exit which does this.

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Note 1: conversion of 1 to bool* has implementation-defined effect ([expr.reinterpret_cast]), using this pointer is implementation-defined ([basic.stc.general]), and comparison of the pointer to nullptr is implementation-defined, though never UB ([expr.eq]).

Note 2: Developers commonly create a local static variable and take its address when they need a std::unique_ptr as a "scope exit object". This is the most robust option.