Why Does Passing a Temporary Object to a Non-Const Reference Fail in C ?

Passing Temporary Objects as References in C

In C , a minimal code snippet such as the following can cause compilation errors on Linux and Mac platforms:

class Foo
{
public:
    Foo(int x) {}
};

void ProcessFoo(Foo& foo)
{
}

int main()
{
    ProcessFoo(Foo(42));
    return 0;
}

Compilation Error:

Compiling the above code results in the following error:

newfile.cpp: In function ‘int main()’:
newfile.cpp:23:23: error: invalid initialization of non-const reference of type ‘Foo&’ from an rvalue of type ‘Foo’
     ProcessFoo(Foo(42));
                       ^
newfile.cpp:14:6: note: in passing argument 1 of ‘void ProcessFoo(Foo&)’
 void ProcessFoo(Foo& foo)

Why the Error Occurs:

By design, C prohibits passing a temporary object to a non-const reference parameter. This restriction is intended to prevent potential misunderstandings and errors. When a function declares a non-const reference parameter, it signifies its intent to modify the object and return it to the caller. Passing a temporary to such a parameter is illogical, as it will be destroyed once the function returns.

Workarounds:

To resolve this compilation error, consider the following workarounds:

• Create a Temporary Variable: Create a non-temporary variable that holds the temporary object before passing it to the function:

Foo foo42(42);
ProcessFoo(foo42);

• Use a Const Reference: Modify the function to take a const reference parameter:

void ProcessFoo(const Foo& foo)

• Pass by Value: Allow the function to take the object as a value parameter:

void ProcessFoo(Foo foo)

Why MSVC Allows It:

It's unclear why Microsoft Visual Studio (MSVC) allows this code to compile while g does not. Potentially, MSVC has a different interpretation of the C standard in this regard. It's generally recommended to follow the standard to ensure consistent behavior across different compilers.

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