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Thinking in C++
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Passing & returning large objects

But now consider user-defined types. If you create a class and you want to pass an object of that class by value, how is the compiler supposed to know what to do? This is not a type built into the compiler; it’s a type you have created.

To investigate this, you can start with a simple structure that is clearly too large to return in registers:

//: C11:PassingBigStructures.cpp
struct Big {
  char buf[100];
  int i;
  long d;
} B, B2;

Big bigfun(Big b) {
  b.i = 100; // Do something to the argument
  return b;

int main() {
  B2 = bigfun(B);
} ///:~

Decoding the assembly output is a little more complicated here because most compilers use “helper” functions instead of putting all functionality inline. In main( ), the call to bigfun( ) starts as you might guess – the entire contents of B is pushed on the stack. (Here, you might see some compilers load registers with the address of the Big and its size, then call a helper function to push the Big onto the stack.)

In the previous code fragment, pushing the arguments onto the stack was all that was required before making the function call. In PassingBigStructures.cpp, however, you’ll see an additional action: the address of B2 is pushed before making the call, even though it’s obviously not an argument. To comprehend what’s going on here, you need to understand the constraints on the compiler when it’s making a function call.

Thinking in C++
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire