Force a C++ class to have non-aligned members

Question

Consider this class:

class C3
{
public:
    uint16_t p1;
    uint16_t p2;
    uint8_t p3;
};

sizeof(C3) is 6. Is there any compiler routine to bypass alignment and force it's size to be 5? I tried to use alignas without success...

Answer 1

With the MSVC compiler (you have included the VS-2015 tag), you can use the #pragma pack directive to set padding options for classes and structures. This also allows you to save/restore 'default' settings, using the push and pop options.

The following short example demonstrates this:

#include <iostream>

class C3 {
public:
    uint16_t p1;
    uint16_t p2;
    uint8_t p3;
};

#pragma pack(push, 1) // Save current packing and set to single-byte
class C4 {
public:
    uint16_t p1;
    uint16_t p2;
    uint8_t p3;
};
#pragma pack(pop) // Restore saved packing

class C5 {
public:
    uint16_t p1;
    uint16_t p2;
    uint8_t p3;
};

int main()
{
    std::cout << sizeof(C3) << std::endl; // 6
    std::cout << sizeof(C4) << std::endl; // 5
    std::cout << sizeof(C5) << std::endl; // 6 (again)
    return 0;
}

Many other compilers support directives equivalent to the MSVC #pragma pack.