CODEWITHSUNDEEP
c++
Divya Aggarwal
Divya Aggarwal

Reputation: 61

When is it advisable to use `size_t` for array declaration or iteration in c++?

Can someone explain and provide examples of situations where using size_t would be more beneficial than, let's say, int or uint8_t for array declarations or iteration?

Upvotes: 1

Views: 487

Answers (3)

MartinBG
MartinBG

Reputation: 1648

While using size_t for iteration seems the natural choice, there is at least one pitfall.

Consider the following for-loop:

int arr[]{0, 1, 2, 3};
for(size_t i = 3; i >=0; --i) {
    std::cout << i << ": " << arr[i] << std::endl;
}

Expected behavior - at least at first glance, is to get these 4 lines printed:

3: 3
2: 2
1: 1
0: 0

What we get instead is something like:

3: 3
2: 2
1: 1
0: 0
18446744073709551615: -1
18446744073709551614: -2
18446744073709551613: 32514
18446744073709551612: -41561408
[and so on]

That's because size_t a = 0; --a; on this particular platform results in a being 18446744073709551615.

On a vague related note, even harder to spot problems can be introduced by mixing of signed and unsigned types in arithmetic operations and some developers state that using data type size_t causes more problems than it solves.

Upvotes: 3

David C. Rankin
David C. Rankin

Reputation: 84569

Match your type to your data. If you have a counter (that can't be negative) or a length or size, ... then an unsigned type makes sense, provides double the range of the same size signed type, etc... For counters, lengths, etc.. the standard C++ sizetype is size_t. Can you use other types -- sure. But in general, size_t is the preferred sizetype.

If you have an value that is size_t and need to iterate over the range, use size_t as your loop counter as well, e.g.

size_t len = mystring.length();

for (size_t i = 0; i < len; i++) { ... }

If you attempt to use a signed loop variable instead, you will generate a Warning: comparison between signed and unsigned types.. So match your types as well.

Upvotes: 7

Paul Evans
Paul Evans

Reputation: 27577

size_t is unsigned and large enough to index any container in memory, it's purpose built to use as an index type.

Upvotes: 2

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