Returning a global object from a function in C++

Question

I'm trying to make a function that returns one of the 3 global objects I have depending on the parameter received, but I'm getting the following error: invalid initialization of non-const reference of type 'block&' from a temporary of type 'block'.

I'm not very familiar with C++ and references, but I tried different things and nothing seems to work. What would be the correct way to do this?

Of course what I want is that the actual chosen global object is passed from getBlock to modifyBlock so that it is modified; at the end the program should print "IT WORKS!".

#include <string>
#include <iostream>

using namespace std;

class block 
{
    public:
    string blockId;
    block (string id);
};

block::block(string id)
{
    blockId=id;
}

block A = block("A");
block B = block("B");

block getBlock (string b)
{
    if(A.blockId==b)
        return A;
    else return B;      
}

block modifyBlock (block &x)
{
    x.blockId="IT WORKS!";
}

int main ()
{
    string b;
    cout<<"Which block? ";
    cin>>b;
    modifyBlock(getBlock(b));
    cout<<endl<<getBlock(b).blockId;
}

Answer 1

This function signature:

block getBlock (string b)

means you are returning a block object by value . You return a copy of one of the global block objects. When used like this:

modifyBlock(getBlock(b));

it fails, because the argument to the function is a temporary, and that cannot bind to a non-const lvalue reference. It looks like you want to return a reference ( block& ) here:

block& getBlock (string b)
{ // ^
  return A.blockId == b ? A : B;
}

Answer 2

Simply modify the function

block getBlock (string b)
{
    if(A.blockId==b)
        return A;
    else return B;      
}

the following way

block & getBlock( const string &b )
{
    if(A.blockId==b)
        return A;
    else return B;      
}

You are going to change either object A or B not their copies are not you?

As for the error then you are passing a temporary object returned by function getBlock to function modifyBlock. Temporary objects my be bound only with constant references.