Thread: About passing reference question

  1. 07-15-2010 #1
    ovid
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    About passing reference question

    I have a question about passing reference
    I have a function prototype like void Queue::enqueue(Const Obj& o);
    The first case, I try to pass a temp Obj instance to a function
    but compile error occurs
    Code:
    Queue q;
q.enqueue(Obj(10));
    I got compile error like following
    g++ -g -Wall -c main.cpp -o main.o
    main.cpp: In function ‘int main()’:
    main.cpp:17: error: no matching function for call to ‘Queue::enqueue(Obj)’
    queue.h:29: note: candidates are: void Queue::enqueue(Obj&)

    Then I try to modify the code as following
    Code:
    Queue q;
Obj o(10);
q.enqueue(o);
    And this one can be compiled
    Could anyone explain what's the difference between these two?
    Thx in advance
  2. 07-15-2010 #2
    laserlight
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    You apparently did not declare enqueue to take the Obj argument by const reference. You declared it to take the Obj argument by non-const reference instead. Besides the obvious difference that for one you can modify the object through the difference, the difference is that with a const reference parameter, you can pass a temporary object, but with a non-const reference parameter you cannot.
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  3. 07-15-2010 #3
    ovid
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    Thx a lot. I am embarrassed I didn't figure out I miss the const specifier.
    However, I didn't know passing a temporary object to a non const reference is illegal while to const one is valid. Learned a lesson. Thx
  4. 07-15-2010 #4
    Elysia
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    Btw, here's another treat for you:
    Code:
    void Queue::enqueue(Obj&& obj) { /* ... */ }

Queue q;
q.enqueue(Obj(10));
    Be sure to enable C++0x mode.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.
  5. 07-15-2010 #5
    cpjust
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    Is that a reference to a reference???
    What would be the point of that?
    "I am probably the laziest programmer on the planet, a fact with which anyone who has ever seen my code will agree." - esbo, 11/15/2008

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  6. 07-15-2010 #6
    Elysia
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    It isn't, It's an r-value reference. A non-const reference that binds to temporaries (not not lvalues unless you use std::move). See wikipedia for more info.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.
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