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Just like classic C++, C++/CLI supports passing parameters by value and by reference. Let s review how this works in classic C++, as in Listing 4-19. Passing a parameter by value means that the function gets a copy of the value, so any operations don t affect the original object. Passing a parameter by reference means that the object is not copied; instead, the function gets the original object, which may consequently be modified. In C++, parameters passed with a reference (&) to an object are passed by reference. That is to say, the object is not copied, and any changes made to the object in the function are reflected in the object after the function returns. Listing 4-19. Passing by Value and by Reference in Classic C++ // parameter_passing.cpp void byvalue(int i) { i += 1; } void byref(int& i) { i += 1; } int main() { int j = 10; System::Console::WriteLine("Original value: " + j); byvalue(j); System::Console::WriteLine("After byvalue: " + j); byref(j); System::Console::WriteLine("After byref: " + j); } The output of Listing 4-19 is Original value: 10 After byvalue: 10 After byref: 11 because only the version that passes the parameter by reference actually affects the value of j in the enclosing scope. Figure 4-2 shows the basic characteristics of passing by value and by reference.
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CHAPTER 4 OBJECT SE MAN TICS IN C++ /CLI
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Figure 4-2. The left side shows the objects in the main method; the right side shows the copies of those values in the function byvalue, and the native reference to the original value in the function byref. Where pointers are involved, the rules are the same, but thinking about them can be a bit trickier. Let s turn the clock back to the time when the C programming language reigned supreme. Consider a somewhat dangerous C function, shown in Listing 4-20, that takes a pointer as a parameter. Listing 4-20. A Dangerous C Function void stringcopy(char* dest, char* src) { while (*dest++ = *src++); } The pointer src is modified within the function, but that does not affect the value outside the function because the pointer is passed by value. In those cases where you need a pointer to be modified, in C, you would use a double pointer (see Listing 4-21). Listing 4-21. Using a Double Pointer in C // double_pointer.cpp #include <malloc.h> int newstring(void** new_buffer) { *new_buffer = malloc( 1024 ); if (! *new_buffer) return -1; return 1; } This is still passing by value, because the address of the pointer is copied. When references were introduced in C++, passing parameters by reference was made possible. For example, the code in Listing 4-22 increments an integer passed in.
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Listing 4-22. Passing by Reference // passing_reference.cpp void increment(int& i) { i++; } If you wanted to pass a pointer by reference in classic C++, you would use *&, a reference to a pointer. void modify_pointer(CClass*& ptr); These constructs have equivalents in the C++/CLI managed world. The handle symbol in the parameter list is used for objects passed by reference. void g(R^ r); This is the normal way of passing reference types. This default makes sense for several reasons. First, passing by value is expensive for larger objects. Primitive types and value types are generally small, and the overhead of passing by value is not large, so value types are usually passed by value, like this: void f(V v); Figure 4-3 shows the typical case of value types and reference types being passed to functions. Because the local data is freed up when the function exists, any changes to the local data, either the local value type or the local handle, are not reflected outside the function. A copy is created of a value type passed to a function, in this case declared as f(V v_local), and a value passed in with the expression f(v). Figure 4-3 also shows a reference type that was passed to a function declared as g(R^ r_local) and a handle passed in with the expression g(r). The local handle in g refers to the same object on the managed heap.
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