Potential FP: returnDanglingLifetime

[Muhammad Javed]

I have a case where I believe cppcheck assesses that a pointer (that gets returned) points to an object that will be destructed upon function exit, which it does not. Here is the sample case:

// test.cpp
#include <memory>
#include <vector>

std::vector<std::unique_ptr<int>> intPtrs;

int* storePtr(int num)
{
    std::unique_ptr<int> localPtr(new int(num));
    int* ret = localPtr.get();
    intPtrs.push_back(std::move(localPtr));
    return ret; // cppcheck reports returnDanglingLifetime here
}

I run cppcheck on the file above and I get:

Checking test.cpp ...
test.cpp:12:12: error: Returning pointer to local variable 'localPtr' that will be invalid when returning. [returnDanglingLifetime]
    return ret;
           ^
test.cpp:10:28: note: Raw pointer to smart pointer created here.
    int* ret = localPtr.get();
                           ^
test.cpp:9:26: note: Variable created here.
    std::unique_ptr<int> localPtr(new int(num));
                         ^
test.cpp:12:12: note: Returning pointer to local variable 'localPtr' that will be invalid when returning.
    return ret;
           ^

Also, it's quite odd that the error says I'm pointing to a local variable when it's actually pointing to the heap-allocated int returned by localPtr.get().

Out of curiosity, I created a behaviorally equivalent portion of code (confirmed by Compiler Explorer) that, oddly, does not reproduce the cppcheck error. This furthers my suspicion that this is a false positive. Here is the equivalent code that does not produce the same error:

// equivalentCase.cpp
#include <memory>
#include <vector>

std::vector<std::unique_ptr<int>> intPtrs;

int* storePtr(int num)
{
    auto localPtr = std::unique_ptr<int>(new int(num));
    int* ret = localPtr.get();
    intPtrs.push_back(std::move(localPtr));
    return ret; // no error here, or anywhere else
}

I only changed how I defined localPtr. I hope this information helps in some way.

I understand the code is a little funky and that I could also use std::make_unique which would probably avoid the error. However, I am running into a case in my project where I am not using std::make_unique and I am instead constructing a std::unique_ptr from an existing pointer. Thanks!

[CHR]

Thanks for reporting, ticket is here: https://trac.cppcheck.net/ticket/11439