There always exists the possibility that the user introduces a value for i so big that the system cannot allocate enough memory for it. This exception method is the method used by default by new, and is the one used in a declaration like: But for now, you should know that if this exception is thrown and it is not handled by a specific handler, the program execution is terminated.
Exceptions are a powerful C++ feature explained later in these tutorials. Using this method, an exception of type bad_alloc is thrown when the allocation fails. Therefore, there are no guarantees that all requests to allocate memory using operator new are going to be granted by the system.Ĭ++ provides two standard mechanisms to check if the allocation was successful: However, computer memory is a limited resource, and it can be exhausted. The dynamic memory requested by our program is allocated by the system from the memory heap. The most important difference is that the size of a regular array needs to be a constant expression, and thus its size has to be determined at the moment of designing the program, before it is run, whereas the dynamic memory allocation performed by new allows to assign memory during runtime using any variable value as size. There is a substantial difference between declaring a normal array and allocating dynamic memory for a block of memory using new. The second element can be accessed either with foo or *(foo+1), and so on. Here, foo is a pointer, and thus, the first element pointed to by foo can be accessed either with the expression foo or the expression *foo (both are equivalent). Therefore, foo now points to a valid block of memory with space for five elements of type int. In this case, the system dynamically allocates space for five elements of type int and returns a pointer to the first element of the sequence, which is assigned to foo (a pointer). The second one is used to allocate a block (an array) of elements of type type, where number_of_elements is an integer value representing the amount of these. The first expression is used to allocate memory to contain one single element of type type. It returns a pointer to the beginning of the new block of memory allocated. new is followed by a data type specifier and, if a sequence of more than one element is required, the number of these within brackets. On these cases, programs need to dynamically allocate memory, for which the C++ language integrates the operators new and delete.ĭynamic memory is allocated using operator new. For example, when the memory needed depends on user input. But there may be cases where the memory needs of a program can only be determined during runtime. In the programs seen in previous chapters, all memory needs were determined before program execution by defining the variables needed.