/Zc:alignedNew (C++17 over-aligned allocation)

Enable support for C++17 over-aligned new, dynamic memory allocation aligned on boundaries greater than the default for the maximum-sized standard aligned type, max_align_t.

Syntax

/Zc:alignedNew[-]

Remarks

The MSVC compiler and library support C++17 standard over-aligned dynamic memory allocation. When the /Zc:alignedNew option is specified, a dynamic allocation such as new Example; respects the alignment of Example even when it's greater than max_align_t, the largest alignment required for any fundamental type. When the alignment of the allocated type is no more than the alignment guaranteed by the original operator new, available as the value of the predefined macro __STDCPP_DEFAULT_NEW_ALIGNMENT__, the statement new Example; results in a call to ::operator new(size_t) as it did in C++14. When the alignment is greater than __STDCPP_DEFAULT_NEW_ALIGNMENT__, the implementation instead obtains the memory by using ::operator new(size_t, align_val_t). Similarly, deletion of over-aligned types invokes ::operator delete(void*, align_val_t) or the sized delete signature ::operator delete(void*, size_t, align_val_t).

The /Zc:alignedNew option is only available when /std:c++17 or later is enabled. Under /std:c++17 or later, /Zc:alignedNew is enabled by default to conform to the C++ standard. If the only reason you implement operator new and delete is to support over-aligned allocations, you may no longer need this code in C++17 or later modes. To turn off this option and revert to the C++14 behavior of new and delete when you use /std::c++17 or later, specify /Zc:alignedNew-. If you implement operator new and delete but you're not ready to implement the over-aligned operator new and delete overloads that have the align_val_t parameter, use the /Zc:alignedNew- option to prevent the compiler and Standard Library from generating calls to the over-aligned overloads. The /permissive- option doesn't change the default setting of /Zc:alignedNew.

Support for /Zc:alignedNew is available starting in Visual Studio 2017 version 15.5.

Example

This sample shows how operator new and operator delete behave when the /Zc:alignedNew option is set.

// alignedNew.cpp
// Compile by using: cl /EHsc /std:c++17 /W4 alignedNew.cpp
#include <iostream>
#include <malloc.h>
#include <new>

// "old" unaligned overloads
void* operator new(std::size_t size) {
    auto ptr = malloc(size);
    std::cout << "unaligned new(" << size << ") = " << ptr << '\n';
    return ptr ? ptr : throw std::bad_alloc{};
}

void operator delete(void* ptr, std::size_t size) {
    std::cout << "unaligned sized delete(" << ptr << ", " << size << ")\n";
    free(ptr);
}

void operator delete(void* ptr) {
    std::cout << "unaligned unsized delete(" << ptr << ")\n";
    free(ptr);
}

// "new" over-aligned overloads
void* operator new(std::size_t size, std::align_val_t align) {
    auto ptr = _aligned_malloc(size, static_cast<std::size_t>(align));
    std::cout << "aligned new(" << size << ", " <<
        static_cast<std::size_t>(align) << ") = " << ptr << '\n';
    return ptr ? ptr : throw std::bad_alloc{};
}

void operator delete(void* ptr, std::size_t size, std::align_val_t align) {
    std::cout << "aligned sized delete(" << ptr << ", " << size <<
        ", " << static_cast<std::size_t>(align) << ")\n";
    _aligned_free(ptr);
}

void operator delete(void* ptr, std::align_val_t align) {
    std::cout << "aligned unsized delete(" << ptr <<
        ", " << static_cast<std::size_t>(align) << ")\n";
    _aligned_free(ptr);
}

struct alignas(256) OverAligned {}; // warning C4324, structure is padded

int main() {
    delete new int;
    delete new OverAligned;
}

This output is typical for 32-bit builds. The pointer values vary based on where your application runs in memory.

unaligned new(4) = 009FD0D0
unaligned sized delete(009FD0D0, 4)
aligned new(256, 256) = 009FE800
aligned sized delete(009FE800, 256, 256)

For information about conformance issues in Visual C++, see Nonstandard Behavior.

To set this compiler option in the Visual Studio development environment

1. Open the project's Property Pages dialog box. For details, see Set C++ compiler and build properties in Visual Studio.

2. Select the Configuration Properties > C/C++ > Command Line property page.

3. Modify the Additional Options property to include /Zc:alignedNew or /Zc:alignedNew- and then choose OK.

See also

/Zc (Conformance)