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Parameters
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It is perfectly legitimate to mix call-by-value and call-by-reference formal parameters in the same function For example, the rst and last of the formal parameters in the following function declaration are call-by-reference formal parameters, and the middle one is a call-by-value parameter:
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void goodStuff(int& par1, int par2, double& par3);
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mixing call-byreference and callby-value parameters
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Call-by-reference parameters are not restricted to void functions You can also use them in functions that return a value Thus, a function with a call-by-reference parameter could both change the value of a variable given as an argument and return a value
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WHAT KIND
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PARAMETER
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Display 43 illustrates the differences between how the compiler treats call-by-value and call-byreference formal parameters The parameters par1Value and par2Ref are both assigned a value inside the body of the function definition Because they are different kinds of parameters, however, the effect is different in the two cases
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par1Value is a call-by-value parameter, so it is a local variable When the function is called as
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follows,
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doStuff(n1, n2);
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the local variable par1Value is initialized to the value of n1 That is, the local variable par1Value is initialized to 1 and the variable n1 is then ignored by the function As you can see from the sample dialogue, the formal parameter par1Value (which is a local variable) is set to 111 in the function body, and this value is output to the screen However, the value of the argument n1 is not changed As shown in the sample dialogue, n1 has retained its value of 1 On the other hand, par2Ref is a call-by-reference parameter When the function is called, the variable argument n2 (not just its value) is substituted for the formal parameter par2Ref So when the following code is executed,
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par2Ref = 222;
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it is the same as if the following were executed:
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n2 = 222;
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Thus, the value of the variable n2 is changed when the function body is executed, so, as the dialogue shows, the value of n2 is changed from 2 to 222 by the function call If you keep in mind the lesson of Display 43, it is easy to decide which parameter mechanism to use If you want a function to change the value of a variable, then the corresponding formal parameter must be a call-by-reference formal parameter and must be marked with the ampersand sign, & In all other cases, you can use a call-by-value formal parameter
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Parameters and Overloading
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Display 43 Comparing Argument Mechanisms
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 //Illustrates the difference between a call-by-value //parameter and a call-by-reference parameter #include <iostream> using namespace std; void doStuff(int par1Value, int& par2Ref); //par1Value is a call-by-value formal parameter and //par2Ref is a call-by-reference formal parameter int main( ) { int n1, n2; n1 = 1; n2 = 2; doStuff(n1, n2); cout << "n1 after function call = " << n1 << endl; cout << "n2 after function call = " << n2 << endl; return 0; } void doStuff(int par1Value, int& par2Ref) { par1Value = 111; cout << "par1Value in function call = " << par1Value << endl; par2Ref = 222; cout << "par2Ref in function call = " << par2Ref << endl; }
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SAMPLE DIALOGUE
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par1Value in function call = 111 par2Ref in function call = 222 n1 after function call = 1 n2 after function call = 222
Pitfall
INADVERTENT LOCAL VARIABLES
If you want a function to change the value of a variable, the corresponding formal parameter must be a call-by-reference parameter and therefore must have the ampersand, &, attached to its type If you carelessly omit the ampersand, the function will have a call-by-value parameter where you meant to have a call-by-reference parameter When the program is run, you will