std::string with a custom allocator

Question

So I'm currently in the process of writing a memory debugger and to do that I need stl container objects to use an untracked allocator.

I have std::string peppered throughout my entire codebase, so I typedef'd it to use my untracked allocator:

typedef std::basic_string<char, std::char_traits<char>, UntrackedAllocator<char>> String;

Now when I try to do this:

String str { "Some string" }; String copy = str;

I get this error:

/usr/local/include/c++/7.1.0/ext/alloc_traits.h:95:67: error: no matching function for call to 'UntrackedAllocator<char>::UntrackedAllocator(UntrackedAllocator<char>)' { return _Base_type::select_on_container_copy_construction(__a); }

This is what my Untracked Allocator looks like:

#pragma once #define NOMINMAX #undef max template <typename T> class UntrackedAllocator { public: typedef T value_type; typedef value_type* pointer; typedef const value_type* const_pointer; typedef value_type& reference; typedef const value_type& const_reference; typedef std::size_t size_type; typedef std::ptrdiff_t difference_type; public: template<typename U> struct rebind { typedef UntrackedAllocator<U> other; }; public: inline explicit UntrackedAllocator() {} inline ~UntrackedAllocator() {} inline explicit UntrackedAllocator(UntrackedAllocator const&) {} template<typename U> inline explicit UntrackedAllocator(UntrackedAllocator<U> const&) {} // address inline pointer address(reference r) { return &r; } inline const_pointer address(const_reference r) { return &r; } // memory allocation inline pointer allocate(size_type cnt, typename std::allocator<void>::const_pointer = 0) { T *ptr = (T*)malloc(cnt * sizeof(T)); return ptr; } inline void deallocate(pointer p, size_type cnt) { free(p); } // size inline size_type max_size() const { return std::numeric_limits<size_type>::max() / sizeof(T); } // construction/destruction inline void construct(pointer p, const T& t) { new(p) T(t); } inline void destroy(pointer p) { p->~T(); } inline bool operator==(UntrackedAllocator const& a) { return this == &a; } inline bool operator!=(UntrackedAllocator const& a) { return !operator==(a); } };

This is my first time working with custom allocators so I have no idea what's going on with it. It's incredibly annoyning that I can't do str1 = str2 if one of them uses a custom allocator.

Your relational operators are wrong. Allocators should compare equal whenever one can deallocate the other's allocation. — Kerrek SB
– Kerrek SB, Commented May 28, 2016 at 19:26
The error message implies that you are using a copy constructor somewhere. The code you posted doesn't use a copy a constructor, though unless your subscript operator returns by value. Anyway, making the copy constructor explicit is rarely useful. Remove the explicit from the copy constructor and things should be fine. — Dietmar Kühl
– Dietmar Kühl, Commented May 28, 2016 at 20:14

Daniel Trugman · Accepted Answer · 2017-10-16 20:12:42Z

The problem is the declaration of the copy c'tors as explicit.

Changing the UntrackedAllocator copy c'tor to:

inline UntrackedAllocator(UntrackedAllocator const&) {}

Solves the compilation issue and everything works just fine:

int main() { String str { "13" }; String copy = str; const char* cstr = str.c_str(); int out = atoi(cstr); }

This happens because the assignment operator of the std::basic_string that accepts const std::basic_string & requires an implicit copy construction of the allocator.

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