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Introduction: parameter passing techniques

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• Review: parameter passing techniques
  • Hopefull, you have learned the 2 most common parameter passing techniques in CS170/CS171:

    Pass-by-value: The value of the parameter variable is passed (= copied) to the method/function Pass-by-reference: The address (= reference) of the parameter variable is passed (= copied) to the method/function

    I said hopefully, because CS170/CS171 uses Java and:

    Java can only pass parameters by value !!!

    Anyway, I like to show you the effect of these dufferent ways to pass parameters in C++

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• The effect of passing a parameter by value
  • Pass-by-value:

    When a parameter variable is passed by value: The program make a copy of the (parameter) variable and give it to the function The function can only update the copy The original variable cannot be updated

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  • Example of pass-bu-value in C++:

    #include <iostream> using namespace std; /* ==================================================== Function with a parameter that is passed by value ==================================================== */ void f(int a) // Var a is passed by value { a = a + 1; } int main(int argc, char *argv[]) { int k = 7; cout << "before f( ): " << k << endl; // Prints: 7 f(k); cout << "after f( ): " << k << endl; // Prints: 7 (k unchanged !) }

    In this program, the function f(a) updates the parameter variable a.

    Due to the fact that the parameter a is passed-by-value, the effect of this update will be:

    The copy (= the parameter variable in f( )) is updated The original variable (= the variable in main( )) is not updated

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/asm/A-pass-by-ref/pass-by-value.C

    How to run the program:

    To compile:   g++ -o pass-by-value pass-by-value.C          To run: pass-by-value

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• The effect of passing a parameter by reference
  • Pass-by-reference:

    When a parameter variable is passed by reference: The program make a copy of the address of the (parameter) variable and give it to the function The function will use the address of the variable to access and update the original variable The original variable can be updated

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  • Example of pass-by-reference in C++:

    #include <iostream> using namespace std; /* ==================================================== Function with a parameter that is passed by value ==================================================== */ void f(int & a) // Var a is passed by reference { a = a + 1; } int main(int argc, char *argv[]) { int k = 7; cout << "before f( ): " << k << endl; // Prints: 7 f(k); cout << "after f( ): " << k << endl; // Prints: 8 (k changed !) }

    In this program, the function f(a) updates the parameter variable a.

    Due to the fact that the parameter a is passed-by-reference, the effect of this update will be:

    The original variable (= the variable in main( )) is updated

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/asm/A-pass-by-ref/pass-by-ref.C

    How to run the program:

    To compile:   g++ -o pass-by-ref pass-by-ref.C          To run: pass-by-ref