Question About Scope

This is a discussion on Question About Scope within the C++ Programming forums, part of the General Programming Boards category; As a C programmer doing C++ stuff a few things are still confusing to me. One is scope. Consider the ...

  1. #1
    Registered User Kernel Sanders's Avatar
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    Question About Scope

    As a C programmer doing C++ stuff a few things are still confusing to me. One is scope. Consider the following code

    Code:
    #include <vector>
    using std::vector;
    
    class A_Class{
     public:
      A_Class(int x);
    // some stuff
    };
    
    void foo(vector<A_Class> *v){
    Class c(42);
    v->push_back(c);
    }
    
    int main(){
     vector<A_Class> *v = new vector<A_Class>();
     foo(v);
     return 0;
    }
    What happens to c and what happens to the vector when foo exits? std::vector<T>.push_back takes as an argument a T&, so my understanding is that the vector contains a reference to c, which is located on foo's stack. When foo returns, c should get nuked, which should screw up the vector. Is that right?

  2. #2
    pronounced 'fib' FillYourBrain's Avatar
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    c is copied into an instance that lives in v. It isn't a reference that lives in v
    "You are stupid! You are stupid! Oh, and don't forget, you are STUPID!" - Dexter

  3. #3
    C++ Witch laserlight's Avatar
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    std::vector<T>.push_back takes as an argument a T&
    No, const T&. push_back() stores a copy of the argument.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  4. #4
    pronounced 'fib' FillYourBrain's Avatar
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    the vector is leaked also, you really don't need to allocate it dynamically. I do realize that's outside the "scope" of the question.... thank you, thank you. I'll be here all week.
    "You are stupid! You are stupid! Oh, and don't forget, you are STUPID!" - Dexter

  5. #5
    dra
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    Also, just fyi about the reference, const T&, it's there to avoid having copy the argument twice, especially if it's a large one.

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