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C++'s Dynamic Allocation Operators
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C++ provides two dynamic allocation operators: new and delete These operators are used to allocate and free memory at run time Dynamic allocation is an important part
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C++: The Complete Reference
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of almost all real-world programs As explained in Part One, C++ also supports dynamic memory allocation functions, called malloc( ) and free( ) These are included for the sake of compatibility with C However, for C++ code, you should use the new and delete operators because they have several advantages The new operator allocates memory and returns a pointer to the start of it The delete operator frees memory previously allocated using new The general forms of new and delete are shown here: p_var = new type; delete p_var; Here, p_var is a pointer variable that receives a pointer to memory that is large enough to hold an item of type type Since the heap is finite, it can become exhausted If there is insufficient available memory to fill an allocation request, then new will fail and a bad_alloc exception will be generated This exception is defined in the header <new> Your program should handle this exception and take appropriate action if a failure occurs (Exception handling is described in 19) If this exception is not handled by your program, then your program will be terminated The actions of new on failure as just described are specified by Standard C++ The trouble is that not all compilers, especially older ones, will have implemented new in compliance with Standard C++ When C++ was first invented, new returned null on failure Later, this was changed such that new caused an exception on failure Finally, it was decided that a new failure will generate an exception by default, but that a null pointer could be returned instead, as an option Thus, new has been implemented differently, at different times, by compiler manufacturers Although all compilers will eventually implement new in compliance with Standard C++, currently the only way to know the precise action of new on failure is to check your compiler's documentation Since Standard C++ specifies that new generates an exception on failure, this is the way the code in this book is written If your compiler handles an allocation failure differently, you will need to make the appropriate changes Here is a program that allocates memory to hold an integer:
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#include <iostream> #include <new> using namespace std; int main() { int *p; try {
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Arrays, Pointers, References, and the Dynamic Allocation Operators
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p = new int; // allocate space for an int } catch (bad_alloc xa) { cout << "Allocation Failure\n"; return 1; } *p = 100; cout << "At " << p << " "; cout << "is the value " << *p << "\n"; delete p; return 0; } C++
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This program assigns to p an address in the heap that is large enough to hold an integer It then assigns that memory the value 100 and displays the contents of the memory on the screen Finally, it frees the dynamically allocated memory Remember, if your compiler implements new such that it returns null on failure, you must change the preceding program appropriately The delete operator must be used only with a valid pointer previously allocated by using new Using any other type of pointer with delete is undefined and will almost certainly cause serious problems, such as a system crash Although new and delete perform functions similar to malloc( ) and free( ), they have several advantages First, new automatically allocates enough memory to hold an object of the specified type You do not need to use the sizeof operator Because the size is computed automatically, it eliminates any possibility for error in this regard Second, new automatically returns a pointer of the specified type You don't need to use an explicit type cast as you do when allocating memory by using malloc( ) Finally, both new and delete can be overloaded, allowing you to create customized allocation systems Although there is no formal rule that states this, it is best not to mix new and delete with malloc( ) and free( ) in the same program There is no guarantee that they are mutually compatible
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