此C ++代码是什么意思

Question

I was trying to understand how webkit parses urls, and I'm having a hard time making heads or tails of this:

Vector<char, 4096> buffer(fragmentEnd * 3 + 1);

This line is on line 1214 (you can see it here: http://trac.webkit.org/browser/trunk/WebCore/platform/KURL.cpp#L1214 ). I get that it's making a vector of type char, with each entry being an array of char 4096 bytes large, but I don't get the buffer(fragmentEnd * 3 + 1) part.

I think that it confuses me most b/c I can't find where the buffer variable is instantiated anywhere (shouldn't it be something more like Vector<char, 4096> buffer = new Vector<char, 4096>(...) ?

Thanks in advance

Answer 1

I get that it's making a vector of type char, with each entry being an array of char 4096 bytes large

It's not. The WTF::Vector template takes two template parameters. The first is the element type (in this case char, which means that each element is a char, not an array of chars). The second is the inline capacity, ie the number of bytes the vector can store without allocating additional memory on the heap.

but I don't get the buffer(fragmentEnd * 3 + 1) part.

The syntax to create a variable on the stack is type variablename(constructor,arguments); . So buffer is the name of the variable and fragmentEnd * 3 + 1 is the argument to Vector<char, 4096> 's constructor (which specifies the vector's initial size).

Answer 2

The buffer variable is constructed exactly on that line:

Vector<char, 4096> buffer(fragmentEnd * 3 + 1);

Walking through the steps that the compiler takes, it first ensures that there are sizeof(Vector<char, 4096>) bytes of space on the stack into which it can construct the Vector<char, 4096> buffer object. It then calls a Vector<char, 4096> constructor that can take an int (because fragmentEnd * 3 + 1 is an int ) on the storage in the stack.

In the current Vector sources, there is no constructor that takes an int . However, there is a constructor that takes a size_t (an unsigned int ), so the compiler will statically cast the result fragmentEnd * 3 + 1 to a size_t and invoke that constructor.

Answer 3

No it shouldn't, new may or may not be used to allocate a new object in C++, unlike, for example, C#.

Explanation:

Vector is not a standard class, like STL's vector.

The line creates object buffer, on stack, of type Vector. It then passes parameters to the constructor (what's in the brackets)

Answer 4

Check out more info about the Vector class that is used (it is the Vector from webkit (WTF namespace) - not from std).

Check the Vector.h header file. It is here .

Answer 5

From Iulian's link, the source for aptly-named WTF::Vector says that the second template parameter is named inlineCapacity .

So, it has something to do with some kind of optimization which should have nothing to do with functionality. Ignore it. The size of the buffer is fragmentEnd * 3 + 1 , nothing more or less.