Difference between revisions of "cpp/memory/c/malloc"

Revision as of 11:44, 12 May 2022

Allocates size bytes of uninitialized storage.

If allocation succeeds, returns a pointer to the lowest (first) byte in the allocated memory block that is suitably aligned for any scalar type (at least as strictly as std::max_align_t).

If size is zero, the behavior is implementation defined (null pointer may be returned, or some non-null pointer may be returned that may not be used to access storage, but has to be passed to std::free)

Parameters

Return value

On success, returns the pointer to the beginning of newly allocated memory. To avoid a memory leak, the returned pointer must be deallocated with std::free() or std::realloc().

On failure, returns a null pointer.

Notes

This function does not call constructors or initialize memory in any way. There are no ready-to-use smart pointers that could guarantee that the matching deallocation function is called. The preferred method of memory allocation in C++ is using RAII-ready functions std::make_unique, std::make_shared, container constructors, etc, and, in low-level library code, new-expression.

For loading a large file, file mapping via OS-specific functions, e.g. mmap on POSIX or CreateFileMapping(A/W) along with MapViewOfFile on Windows, is preferable to allocating a buffer for file reading.

Example

#include <iostream>   
#include <cstdlib> 
#include <string>
#include <memory>
 
int main() 
{
    constexpr std::size_t size = 4;
    if (auto ptr = reinterpret_cast<std::string*>(
            std::malloc(size * sizeof(std::string))))
    {   try
        {   for (std::size_t i = 0; i < size; ++i)
                std::construct_at(ptr + i, 5, 'a' + i);
            for (std::size_t i = 0; i < size; ++i)
                std::cout << "ptr[" << i << "] == " << ptr[i] << '\n';
            std::destroy_n(ptr, size);
        }
        catch(...) {}
        std::free(ptr);
    }
}

p[0] == aaaaa
p[1] == bbbbb
p[2] == ccccc
p[3] == ddddd

See also