Proper way to create unique_ptr that holds an allocated array

Question

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]());      memset(testData.get(),0x12,0);      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/vgpreload_memcheck-amd64-linux.so) ==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/vgpreload_memcheck-amd64-linux.so) ==3894==    by 0x40075F: main (test.cpp:15) 

Answer 1

Using the T[] specialisation:

std::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 provides you with the std::make_unique function template. See this excellent GOTW for more details. The syntax is:

auto testData = std::make_unique<unsigned char[]>(16000);