#include <windows.h>
#include <cstdio>
using namespace std;

const unsigned char OP_JMP = 0xE9;  // 32 bit relative jmp
const SIZE_T SIZE_PATCH = 5;        // jmp dword ptr distance; 1 byte + 4 bytes
typedef void (*MyProc)();

void SimpleFunction1()
{
    printf("foo\n");
}

void SimpleFunction2()
{
    printf("bar\n");
}

int main()
{
    PBYTE foo = reinterpret_cast<PBYTE>(SimpleFunction1);
    PBYTE bar = reinterpret_cast<PBYTE>(SimpleFunction2);

    DWORD oldProtection;
    // make sure the bytes of the function are writable
    // by default they are only readable and executable
    BOOL res = VirtualProtect(foo, SIZE_PATCH, PAGE_EXECUTE_READWRITE, &oldProtection);
    if (!res) return 1;

    // be mindful of pointer arithmetic
    // works with PBYTE, won't with PDWORD
    DWORD distanceToNewFoo = bar - foo - SIZE_PATCH;

    *foo = OP_JMP;
    *reinterpret_cast<PDWORD>(foo + 1) = distanceToNewFoo;

    // called though the pointer instead of foo()
    // to make sure the compiler won't inline or do some other stupid stuff
    reinterpret_cast<MyProc>(foo)(); // will print "bar\n"
    return 0;
}