std::realloc

Header: <cstdlib>

Reallocates the given area of memory (implicitly creating objects in the destination area). It must be previously allocated by std::malloc, std::calloc or std::realloc and not yet freed with std::free, otherwise, the results are undefined.

# Declarations

void* realloc( void* ptr, std::size_t new_size );

# Parameters

# Return value

On success, returns a 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. The original pointer ptr is invalidated and any access to it is undefined behavior (even if reallocation was in-place).

# Notes

Because reallocation may involve bytewise copying (regardless of whether it expands or contracts the area), it is necessary (but not sufficient) for those objects to be of TriviallyCopyable type.

Some non-standard libraries define a type trait “BitwiseMovable” or “Relocatable”, which describes a type that does not have:

Objects of such type can be accessed after their storage is reallocated even if their copy constructors are not trivial.

# Example

#include <cassert>
#include <cstdlib>
#include <new>
 
class MallocDynamicBuffer
{
    char* p;
public:
    explicit MallocDynamicBuffer(std::size_t initial = 0) : p(nullptr)
    {
        resize(initial);
    }
 
    ~MallocDynamicBuffer() { std::free(p); }
 
    void resize(std::size_t newSize)
    {
        if (newSize == 0) // this check is not strictly needed,
        {
            std::free(p); // but zero-size realloc is deprecated in C
            p = nullptr;
        }
        else
        {
            if (void* mem = std::realloc(p, newSize))
                p = static_cast<char*>(mem);
            else
                throw std::bad_alloc();
        }
    }
 
    char& operator[](size_t n) { return p[n]; }
    char operator[](size_t n) const { return p[n]; }
};
 
int main()
{
    MallocDynamicBuffer buf1(1024);
    buf1[5] = 'f';
    buf1.resize(10); // shrink
    assert(buf1[5] == 'f');
    buf1.resize(1024); // grow
    assert(buf1[5] == 'f');
}

# See also