gcc object passing by value or by address

Question

We have this simple class:

class MyStr
{
public:
    MyStr() { }
    wchar_t* pStr;
};

and this code that uses it:

MyStr ms1, ms2;
ms1.pStr = L"MyStr!"; 
ms2.pStr = L"MyStr2!";

wchar_t buffer[100];
swprintf(buffer, 100, L"%ls - %ls", ms1, ms2);

The assembly that pushes ms1 and ms2 onto the stack looks like:

FF B5 24 FE FF FF    push DWORD PTR [ebp-0x1dc] 
FF B5 20 FE FF FF    push DWORD PTR [ebp-0x1e0] 

It is actually pushing the the values/content of MyStr (which in this case is just pStr) onto the stack.

If we change MyStr to just add a simple destructor:

class MyStr
{
public:
    MyStr() { }
    ~MyStr() { }
    wchar_t* pStr;
};

now the address of ms1 and ms2 are getting passed, instead of their values/contents.

8D 85 24 FE FF FF    lea eax,[ebp-0x1dc] 
50                   push eax 
8D 85 20 FE FF FF    lea eax,[ebp-0x1e0] 
50                   push eax

Visual Studio on Windows gives the same result both ways (always passes values/contents), but gcc on Linux gives these two different results.

1. Why?
2. What can we do to keep the destructor but pass by value instead of by reference/address?

The thing we cannot do is change the swprintf line - there are many thousands of them and we are trying to avoid changing them.

Answer 1

You may resort to MACRO

// Forward call to your method
#define swprintf(...) my_swprintf(__VA_ARGS__)

// Your class
class MyStr
{
public:
    MyStr() { }
    ~MyStr() { }
    const wchar_t* pStr;
};


// helper function to "fix" parameters
template <typename T>
std::enable_if_t<std::is_arithmetic<T>::value || std::is_pointer<T>::value, T>
normalize(T t) { return t; } // Default one which does nothing

// Fix for your class to return its member.
const wchar_t* normalize(const MyStr& t) { return t.pStr; }

// Your function which (re-)forward to real swprintf with fixed parameter
// Extra parent do avoid macro substitution
template <typename ... Ts>
auto my_swprintf(wchar_t* buffer, const wchar_t* format, const Ts&... args)
{
    return (swprintf)(buffer, format, normalize(args)...);
}