How to get confused by VC++

timoch — Wed, 27 Mar 2013 14:17:34 +0000

Today, I came across a confusing compilation error with MS VC++ using a std::unique_ptr with a custom deleter.

Here the code snippet:

#include 
#include 

typedef void (*void_delete)(void *);

template
void void_deleter(void * ptr) { delete reinterpret_cast(ptr); }

std::unique_ptr f1() {
return std::unique_ptr(new std::string("haha"), void_deleter);
}
std::unique_ptr f2() {
return std::unique_ptr(nullptr);
}

The compiler complains :

1
2
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9
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13

c:\program files (x86)\microsoft visual studio 10.0\vc\include\memory(2218): error C2338: unique_ptr constructed with null deleter pointer
1> c:\program files (x86)\microsoft visual studio 10.0\vc\include\memory(2214) : while compiling class template member function 'std::unique_ptr::unique_ptr(std::nullptr_t)'
1> with
1> [
1> _Ty=void,
1> _Dx=void_delete
1> ]
1> c:\sources\source1.cpp(9) : see reference to class template instantiation 'std::unique_ptr' being compiled
1> with
1> [
1> _Ty=void,
1> _Dx=void_delete
1> ]

The gotcha is that the compiler complains of a reference to class template instantiation on line 9. This is the definition of f1().

However, the actual issue is on line 13 in the implementation of f2(). A std::unique_ptr is instantiated with a nullptr and no deleter. The nullptr constructor has a static_assert that protects from uninitialized function-pointer deleter. I wonder what could go wrong with an uninitialized function pointer …

The compiler gives you the line of the first appearance of std::unique_ptr in the compilation unit not the actual line that causes the error … Easy enough to spot on a trimmed down example like this one. It tooks me quite some time to find in a code file with several hundred lines of code and many changes at once.

TiMoch » visual studio

How to get confused by VC++