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Dynamic memory allocation and deallocation functions can be globally replaced by custom implementations, as specified by [replacement.functions], paragraph 2, of the C++ Standard [ISO/IEC 14882-2014]. For instance, a user may profile the dynamic memory usage of an application and decide that the default allocator is not optimal for their usage pattern, and a different allocation strategy may be a marked improvement. However, the C++ Standard, [res.on.functions], paragraph 1, states the following:

In certain cases (replacement functions, handler functions, operations on types used to instantiate standard library template components), the C++ standard library depends on components supplied by a C++ program. If these components do not meet their requirements, the Standard places no requirements on the implementation.

Paragraph 2 further states, in part, states the following:

In particular, the effects are undefined in the following cases:
— for replacement functions, if the installed replacement function does not implement the semantics of the applicable Required behavior: paragraph.

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This compliant solution implements the required behavior for the replaced global allocator function by properly throwing a std::bad_alloc exception when the allocation fails:.

#include <new>

void *operator new(std::size_t size) {
  extern void *alloc_mem(std::size_t); // Implemented elsewhere; may return nullptr
  if (void *ret = alloc_mem(size)) {
    return ret;
  }
  throw std::bad_alloc();
}
 
void operator delete(void *ptr) noexcept; // Defined elsewhere
void operator delete(void *ptr, std::size_t) noexcept; // Defined elsewhere

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Failing to meet the stated requirements for a replaceable dynamic storage function leads to undefined behavior. The severity of risk depends heavily on the caller of the allocation functions, but in some situations, dereferencing a null pointer can lead to the execution of arbitrary code [Jack 2007], [van Sprundel 2006]. The indicated severity is for this more severe case.

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