Vector. Resize? And? What is the difference between vector. Reserve ?. XML

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Reserve is a container reserved space, but does not actually create element objects. Before creating an object, you cannot reference the elements in the container. Therefore, when adding a new element, you must use push_back () /insert () function. Resize is used to change the container size and create objects. Therefore, after calling this function, you can reference objects in the container. Therefore, when a new element is added, use the operator [] operator or the iterator to reference element objects.

Furthermore, there are differences between the two functions. The reserve function is followed by a parameter, that is, the space of the container to be reserved. The resize function can have two parameters, the first parameter is the new container size, and the second parameter is to add new elements in the container. If this parameter is omitted, the default constructor of the element object is called. The following are examples of the two functions:

???? Vector <int>? Myvec ;???? Myvec. Reserve (? 100 ?);????? //? The new element has not yet been constructed ,?????????????????????????????? //? In this case, the [] access element ???? For? (Int? I? =? 0 ;? I? <? 100 ;? I ++ ?)????... {????????? Myvec. push_back (? I ?);? // The new element is constructed at this time ????}???? Myvec. Resize (? 102 ?);?????? //? Two new elements are constructed using the default constructor of the element ???? Myvec [2, 100]? =? 1 ;??????????? // Directly operate the new element ???? Myvec [2, 101]? =? 2 ;? The first contact with these two interfaces may be confused. In fact, the interface name is an excellent description of the function, resize is to re-allocate the size, and reserve is to reserve a certain amount of space. The two interfaces have differences and are similar. The following is an analysis of their details .????? To implement the resize semantics, the resize interface provides two guarantees :???????????? First, ensure that the range is [0 ,? New_size) data within the range is valid. If the subscript index is in this range, vector [indext] is valid .????????????? The second is to ensure that the range is [0 ,? Data Out Of The new_size range is invalid. If the subscript index is out of the range, vector [indext] is invalid .????? Reserve only ensures that the space (capacity) of a vector is at least N of the size specified by its parameter. In the range [0 ,? N) in the range, if the subscript is index, the access to vector [Index] may be legal or illegal, depending on the actual situation .????? The common feature of the resize and reserve interfaces is that they both ensure that the space size (capacity) of the vector reaches the minimum size specified by its parameter.

Because the source code of the two interfaces is quite simplified, You can paste them here:

?? Void? Resize (size_type? New_size )? {? Resize (new_size ,? T ());?}?? Void? Resize (size_type? New_size ,? Const? T &? X )? {???? If? (New_size? <? Size ())?????? Erase (begin ()? +? New_size ,? End ());? //? Ensure that data outside the erase range is invalid ???? Else ?????? Insert (end (),? New_size? -? Size (),? X );? //? Fill in the vacant data within the range to ensure that the data within the range is valid

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Vector. Resize? And? What is the difference between vector. Reserve ?. XML