lauw lauw - 2 years ago 151
Linux Question

Proper way to create unique_ptr that holds an allocated array

What is the proper way to create an unique_ptr that holds an array that is allocated on the free store? Visual studio 2013 supports this by default, but when I use gcc version 4.8.1 on Ubuntu I get memory leaks and undefined behaviour.

The problem can be reproduced with this code:

#include <memory>
#include <string.h>

using namespace std;

int main()
unique_ptr<unsigned char> testData(new unsigned char[16000]());


return 0;

Valgrind will give this output:

==3894== 1 errors in context 1 of 1:
==3894== Mismatched free() / delete / delete []
==3894== at 0x4C2BADC: operator delete(void*) (in /usr/lib/valgrind/
==3894== by 0x400AEF: std::default_delete<unsigned char>::operator()(unsigned char*) const (unique_ptr.h:67)
==3894== by 0x4009D0: std::unique_ptr<unsigned char, std::default_delete<unsigned char> >::~unique_ptr() (unique_ptr.h:184)
==3894== by 0x4007A9: main (test.cpp:19)
==3894== Address 0x5a1a040 is 0 bytes inside a block of size 16,000 alloc'd
==3894== at 0x4C2AFE7: operator new[](unsigned long) (in /usr/lib/valgrind/
==3894== by 0x40075F: main (test.cpp:15)

Answer Source

Using the T[] specialisation:

unique_ptr<unsigned char[]> testData(new unsigned char[16000]());

Note that, in an ideal world, you would not have to explicitly use new to instantiate a unique_ptr, avoiding a potential exception safety pitfall. To this end, C++14 will provide you with the std::make_unique function template. See this excellent GOTW for more details.

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