Clareza no código: uso de macro e resultado.

#include<iostream>
#include<cstdlib>

typedef struct estrutura{
    char caracter;  int inteiro;
} estrutura;

#define ESTRUTURA(index) ( (estrutura *) buffer) [ (long long) (index) ]

inline int soma( int i1, int i2) { return i1 + i2; }

int main( void){
    void *buffer;

    buffer = (void *) malloc( sizeof( estrutura) * 2);

    ESTRUTURA( 0).caracter = 10;   ESTRUTURA( 1).inteiro = 15;

// COM MACRO
    std::cout << soma( ESTRUTURA(0).caracter, ESTRUTURA(1).inteiro) << std::endl;

// SEM MACRO (MESMO QUE ACIMA)
    std::cout << soma( (( (estrutura *) buffer) [ 0]).caracter,
                       (( (estrutura *) buffer) [ 1]).inteiro ) << std::endl;
    free( buffer);
    return 0;
}

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CÓDIGO GERADO PELO COMPILADOR (TDM-GCC 4.7.1 32 bits, Orwell Dev C++ Windows)

COM MACRO:

   0x0040153b <+59>:    add    eax,0x8
   0x0040153e <+62>:    mov    edx,DWORD PTR [eax+0x4]
   0x00401541 <+65>:    mov    eax,DWORD PTR [ebp-0xc]
   0x00401544 <+68>:    movzx  eax,BYTE PTR [eax]
   0x00401547 <+71>:    movsx  eax,al
   0x0040154a <+74>:    mov    DWORD PTR [esp+0x4],edx
   0x0040154e <+78>:    mov    DWORD PTR [esp],eax
   0x00401551 <+81>:    call   0x4157e0 <soma(int, int)>
   0x00401556 <+86>:    mov    DWORD PTR [esp],eax
   0x00401559 <+89>:    mov    ecx,0x47bf60
   0x0040155e <+94>:    call   0x4450f0 <std::ostream::operator<<(int)>
   0x00401563 <+99>:    sub    esp,0x4
   0x00401566 <+102>:   mov    DWORD PTR [esp],0x46dd30
   0x0040156d <+109>:   mov    ecx,eax
   0x0040156f <+111>:   call   0x444df0 <std::ostream::operator<<(std::ostream& (*)(std::ostream&))>
   0x00401574 <+116>:   sub    esp,0x4
   0x00401577 <+119>:   mov    eax,DWORD PTR [ebp-0xc]


SEM MACRO:

   0x0040157a <+122>:   add    eax,0x8
   0x0040157d <+125>:   mov    edx,DWORD PTR [eax+0x4]
   0x00401580 <+128>:   mov    eax,DWORD PTR [ebp-0xc]
   0x00401583 <+131>:   movzx  eax,BYTE PTR [eax]
   0x00401586 <+134>:   movsx  eax,al
   0x00401589 <+137>:   mov    DWORD PTR [esp+0x4],edx
   0x0040158d <+141>:   mov    DWORD PTR [esp],eax
   0x00401590 <+144>:   call   0x4157e0 <soma(int, int)>
   0x00401595 <+149>:   mov    DWORD PTR [esp],eax
   0x00401598 <+152>:   mov    ecx,0x47bf60
   0x0040159d <+157>:   call   0x4450f0 <std::ostream::operator<<(int)>
   0x004015a2 <+162>:   sub    esp,0x4
   0x004015a5 <+165>:   mov    DWORD PTR [esp],0x46dd30
   0x004015ac <+172>:   mov    ecx,eax
   0x004015ae <+174>:   call   0x444df0 <std::ostream::operator<<(std::ostream& (*)(std::ostream&))>
   0x004015b3 <+179>:   sub    esp,0x4
=> 0x004015b6 <+182>:   mov    eax,DWORD PTR [ebp-0xc]