What is the size of a pointer? [duplicate]

This question already has answers here: Do all pointers have the same size in C++? (10 answers) Closed 6 months ago.

Is the size of a pointer the same as the size as the type it's pointing to, or do pointers always have a fixed size? For example...

int x = 10;
int * xPtr = &x;
char y = 'a';
char * yPtr = &y;

std::cout << sizeof(x) << "\n";
std::cout << sizeof(xPtr) << "\n";
std::cout << sizeof(y) << "\n";
std::cout << sizeof(yPtr) << "\n";

Wh开发者_StackOverflow中文版at would the output of this be? Would sizeof(xPtr) return 4 and sizeof(yPtr) return 1, or would the 2 pointers actually return the same size?

The reason I ask this is because the pointers are storing a memory address and not the values of their respective stored addresses.

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Function Pointers can have very different sizes, from 4 to 20 bytes on an x86 machine, depending on the compiler. So the answer is no - sizes can vary.

Another example: take an 8051 program. It has three memory ranges and thus has three different pointer sizes, from 8 bit, 16 bit, 24 bit, depending on where the target is located, even though the target's size is always the same (e.g., char).

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Pointers generally have a fixed size, for ex. on a 32-bit executable they're usually 32-bit. There are some exceptions, like on old 16-bit windows when you had to distinguish between 32-bit pointers and 16-bit... It's usually pretty safe to assume they're going to be uniform within a given executable on modern desktop OS's.

Edit: Even so, I would strongly caution against making this assumption in your code. If you're going to write something that absolutely has to have a pointers of a certain size, you'd better check it!

Function pointers are a different story -- see Jens' answer for more info.

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On 32-bit machine sizeof pointer is 32 bits ( 4 bytes), while on 64 bit machine it's 8 byte. Regardless of what data type they are pointing to, they have fixed size.

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To answer your other question. The size of a pointer and the size of what it points to are not related. A good analogy is to consider them like postal addresses. The size of the address of a house has no relationship to the size of the house.

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Pointers are not always the same size on the same architecture.

You can read more on the concept of "near", "far" and "huge" pointers, just as an example of a case where pointer sizes differ...

http://en.wikipedia.org/wiki/Intel_Memory_Model#Pointer_sizes

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They can be different on word-addressable machines (e.g., Cray PVP systems).

Most computers today are byte-addressable machines, where each address refers to a byte of memory. There, all data pointers are usually the same size, namely the size of a machine address.

On word-adressable machines, each machine address refers instead to a word larger than a byte. On these, a (char *) or (void *) pointer to a byte of memory has to contain both a word address plus a byte offset within the addresed word.

http://docs.cray.com/books/004-2179-001/html-004-2179-001/rvc5mrwh.html

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Recently came upon a case where this was not true, TI C28x boards can have a sizeof pointer == 1, since a byte for those boards is 16-bits, and pointer size is 16 bits. To make matters more confusing, they also have far pointers which are 22-bits. I'm not really sure what sizeof far pointer would be.

In general, DSP boards can have weird integer sizes.

So pointer sizes can still be weird in 2020 if you are looking in weird places

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The size of a pointer is the size required by your system to hold a unique memory address (since a pointer just holds the address it points to)