Want to know the length of a buffer C++

Question

I am new to C++ and programming and I would like to know if there is a way to get the length of a pointer.
Let's say Myheader is a struct with different types of data inside.
My code goes like this:

char *pStartBuffer;
memcpy(pStartBuffer, &MyHeader, MyHeader.u32Size);

So I want to know the length of the buffer so that I can copy the data to a file using QT write function.

file.write(pStartBuffer, length(pStartBuffer));

How can I do this?

And no, there's no standard way of knowing the length of the memory a pointer points to. You have to keep track of it yourself. — Some programmer dude, Jan 29 '15 at 15:03
Don't use a `char` buffer in the first place. Use `std::string` and none of this will even be a problem. — John Dibling, Jan 29 '15 at 15:14

score 1 · Answer 1 · answered Jan 29 '15 at 15:06

1

There is no way to find the length of a buffer given nothing but a pointer. If you are certain that it's a string you can use one of the string length functions, or you can keep track of the length of the buffer yourself.

answered Jan 29 '15 at 15:06

SubSevn

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eerorika · Answer 2 · 2015-01-29T15:16:18.753

Pointer doesn't have a "length". What you need is the length of the array which the pointer points to.

No, you cannot extract that information from the pointer.

If the array contains a valid, null-terminated character string, then you can get the length of that string by iterating it until you find a null character which is what strlen does.

If not, then what you normally do is you store the length in a variable when you allocate the array. Which is one of the things that std::vector or std::string will do for you, whichever is more appropriate for your use.

score 1 · Accepted Answer · answered Jan 29 '15 at 15:11

Pointers don't know its allocated size.

You may use std::vector which keep track of size for you:

std::vector<char> pStartBuffer(MyHeader.u32Size);
memcpy(pStartBuffer, &MyHeader, MyHeader.u32Size);

And latter:

file.write(pStartBuffer.data(), pStartBuffer.size());

Want to know the length of a buffer C++

3 Answers3