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Pass by value means the value of the actual argument is assigned to the formal argument of the function. This is how all functions work in all languages, but in some languages, such as Java, the implementation details may be hidden from us so it may not be obvious pass by value is occurring. When we say pass by reference what we really mean is that the value being passed is a memory address rather than an ordinary value. That is; the memory address is being passed by value. Memory addresses can only be stored in pointers, so for a function to accept an address its formal argument must be pointer of the same type. In languages that do not support pointers, such as Java, the pointers are still there we just don't have access to them.
In C++ we also have a native reference type. A reference is nothing more than an alias, an alternative name for an object of the same type. Unlike a pointer, a reference cannot change which object it refers to; it always refers to the same object it was assigned when it came into scope. Moreover, a reference can never be null. As a result, references are much easier to work with than pointers because we know that an object must exist if we have a reference. With pointers, we must test the pointer is non-null before we can determine if it refers to an actual object. In addition, a pointer has storage and identity whereas a reference does not. If we attempt to take the address of a reference, we take the address of the object being referred to. If we take the address of a pointer we get the pointer's own address (as opposed to the address stored at that address).
Although C++ supports a native reference type, Behind the Scenes the compiler uses pointers to implement them. So references are really just passed by value the same way pointers are passed by value. Regardless, if a function accepts either a pointer or a reference, it is said to be using the pass by reference semantic, otherwise it is using the (default) pass by value semantic.
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How can you change values in call by reference?
We don't call by reference, we call functions. The arguments passed to the function are passed (not called) either by value or by reference, depending upon the function signature (the prototype). When you pass by reference you are passing the actual variable, not a copy of the variable, thus the function can modify that variable's value directly. The only exception is when the parameter is declared a constant reference. Passing a pointer is essentially the same as passing by reference, however the pointer itself is passed by value. To pass a pointer by reference you must pass a pointer-to-pointer instead. Passing by value always copies the value, whether it is declared constant or not. But if it is declared constant, the function might as well accept a constant reference. Passing objects (instances of a class) by constant value will incur a performance penalty in making an unnecessary copy. If it is constant, there is little point in copying the object.
What is a variable that is used within a function?
If the variable is declared within the function body, it is a local variable, one that is local to the function. Local variables fall from scope when the function returns, they are only accessible within the function. However, local variables can be returned by value, which creates an automatic variable that is returned to the caller. If the caller does not store the return value, the automatic variable falls from scope when the expression containing the function call ends. However, the expression may evaluate the return value without storing it. Note that functions cannot return local variables by reference since the local variable falls from scope when the function returns. If the variable is passed as an argument to the function, then the variable is a parameter of the function. Arguments may be passed by value or by reference, depending upon the function signature. Passing by value means the function parameter is a copy of the argument (if the argument is an object, the object's copy constructor is invoked automatically). Thus any changes made to the parameter within the function are not reflected in the argument that was originally passed, and the parameter will fall from scope when the function returns. However, the value of the parameter can be returned as previously explained. Passing by reference means the function parameter refers directly to the argument that was passed. Thus any changes made to the parameter are reflected in the argument. Parameters that are declared as constant references assure the caller that the reference's immutable members will not be altered by the function. If the parameter is a non-const reference but the caller does not wish changes to be reflected in the argument, the caller should pass a copy of the argument instead.
What are the different parameter passing methods used by c plus plus?
Pass by value, constant value, reference and constant reference. Pass by value is the default in C++ (pass by reference is the default in Java).
Difference between pass by value and pass by reference?
When you pass by value you pass a copy of the name. That is, the function's parameter is assigned the value of the name you pass. If the function parameter is a class type, then that class' copy constructor is called, automatically, which assigns the name's member value(s) to a new instance of the class. The function parameter is temporary, it has local scope, and will therefore fall from scope when the function returns. Thus any changes made to the local name within the function have no effect on the name you passed -- they are completely separate names. When you pass by reference you pass the name itself. To be precise, you pass the memory address of the name, its reference. This address is assigned to the function's reference parameter. Thus any changes made to the reference parameter's value will change the name you passed, since they both refer to the same memory address. Passing by reference is the most efficient method of passing objects (instances of a class) to functions as there is no overhead in copying the object. And if a reference parameter is declared constant then you can be assured the function will not alter the immutable members of the object (while there are ways around this, if a function really intends to alter a reference parameter then it should not be declared constant in the first place). Pass by value should only be used when the value is a primitive data type (generally equal to or smaller than a pointer), or when the function needs to alter the value but must not alter the name that was passed. Note that pointers are always passed by value, but since they can store a memory address they can be treated just as if they were references (assuming the pointer is non-zero of course). When passing pointers, the pointer and the address being pointed at can both be independently declared constant (that is, either, both, or neither can be constant). To pass a pointer by reference you must pass a pointer-to-pointer type, which is itself passed by value. This allows the function to not only modify the address being pointed at (making it point to another address), but to also modify the value at that address, just as if you'd passed the value by reference.
What are the possibe problems if you use call by value instead of call by reference?
When you pass by value you essentially pass a temporary copy of the value. If the value's parameter is declared const, then copying the value could be costly, especially if the value is a large and complex structure or object. If the value's parameter is non-const, then it has to be assumed the function intends to alter the value in some way. If you pass by value, only the copy will be affected, not the original value. When a parameter is declared constant, passing by reference is generally the way to go. When it is non-const, pass by reference if you fully expect any changes to be reflected in the original value, otherwise pass by value.
Do functions get called by reference only?
No. Function parameters are passed by value. Always. Even the so called "call by reference" is a value - the value of the pointer or the address of the object - but what is placed in the parameter list is a value.
What is the difference between passing by value and passing by difference?
Passing by value is where you pass the actual value (be it an integer, an array, a struct, etc.) to a function. This means that the value must be copied before the function is called and therefore the value in the caller cannot be modified within the callee. Passing by reference is where a reference/pointer to a value is passed to a function. The reference often takes up less space than copying the actual value (particularly when the value is a struct or an array) and allows you to manipulate the value in the caller within the callee.
What is parameters in C plus plus?
Parameters are the formal arguments of a function, as defined by the function. When you pass arguments to a function, those arguments are assigned to the function's parameters, either by value or by reference, depending on how the parameters are declared in the function. The following example explains both: void foo( int param ) { // param is a by value parameter, which is a copy of the argument passed to it. } void bar( int& param ) { // param is a reference parameter, which references the argument passed to it. } int main() { int arg = 100; foo( arg ); bar( arg ); return( 0 ); } Note that passing a pointer is the same as passing an int by value: the pointer's value is passed to the function, not the pointer itself. To pass a pointer by reference, you must pass a pointer to pointer and the function's parameter must accept a pointer to pointer.
Why printf is known as call by value?
C language uses only one method for parameter-passing: call by value.
What are the advantages of pass by reference as compared to pass by value?
Call by reference, particularly when applied to objects, because call by value automatically invokes an object's copy constructor, which is seldom desirable when passing objects into functions.
What is Call by value?
when the function is call by value,u are making any changes in formal parameter does not reflect the actuasl parameter.
What is a reference variable in c plus plus?
A reference variable in C++ is a formal parameter of a function call that automatically dereferences itself, as if it were a pointer, into a reference to the original value in the calling routine. You declare the reference type in the function declaration and prototype, but the compiler automatically adds the reference (&) operator on call, and the dereference (*) operator on use.
