What is std::vector::front() used for?

Sorry if this has bee开发者_运维知识库n asked before, but I am wondering what the use of std::vector::front() is.

Is there a reason to use e.g. myvector.front() rather than myvector[0] or myvector.at(0)?

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Some of the generic algorithms that also work on lists use it.

This is an example of a general principle: if you provide accessors for all the semantics you support, not just the implementation you support, it is easier to write generically and therefore easier to reuse code.

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If the type of myvector changes to another data type that is not indexable, such as a list, you will not have to change code that accesses the front of the container.

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Doing this provides something called static polymorphism.

Let's say I've written an algorithm using a queue class. It has a front() function to get the next element of the queue, and an enqueue() function to add to the end of the queue. Now let's say I discovered that this queue class is written poorly and very slow, and I'd rather use std::vector which is much faster (I know there's a std::queue, this is just an example). If the only way to get the first element of a std::vector was with v[0], I'd have to go through my code and replace all my calls to front() with [0]. But by implementing front(), std::vector can now be a drop-in replacement for my queue class. The only code I have to change is the type of the container in my algorithm.