initializing array with variable

Question

I'm having an issue initializing std::array with variable.

std::string str = "Hello world this is just testing";

int size = str.size();

std::array<char, size> arr;

Returns following error:

note: 'int size' is not const
error: the value od 'size' is not usable in a constant expression
note: in template argument for type 'long long unsigned int'

I'm newbie, please help.

Just like for plain C-style arrays, the size must be a compile-time constant (this is actually a requirement for all template value arguments). If you want an "array" whose size is known only at run-time you must use std::vector. — Some programmer dude
– Some programmer dude, Commented Mar 30, 2022 at 8:21
I'm also very curious why you need an array (or vector) with the same size as an existing string? What is the use of the array (or vector)? What problem is it supposed to solve? Can't you use the string itself? — Some programmer dude
– Some programmer dude, Commented Mar 30, 2022 at 8:23
Thanks @Someprogrammerdude, I couldn't think of std::vector, saved my time. — Dennis Earlyman
– Dennis Earlyman, Commented Mar 30, 2022 at 8:27
As long as you're working with text, use std::string. Everything you can do with a std::vector, you can also do with a std::string. — Some programmer dude
– Some programmer dude, Commented Mar 30, 2022 at 8:53

eerorika · Accepted Answer · 2022-03-30 08:29:05Z

4

You cannot use variables as template arguments unless they are compile time constant. Hence, the size of std::array must be compile time constant. What you're trying to do is not possible. Same limitation applies to array variables as well.

std::string already internally owns a dynamic array of char, so you probably don't need std::array at all.

answered Mar 30, 2022 at 8:29

eerorika

240k13 gold badges212 silver badges354 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

saleh sargolzaee · Accepted Answer · 2022-03-30 12:08:57Z

1

You need to pass a constant expression as the size to the array data structure. If you want to assign variable length to a data structure, you can use std::vector<char> v(size); instead of an array. I also think you can use variable length with arrays if you use the GCC compiler

edited Mar 30, 2022 at 12:08

answered Mar 30, 2022 at 8:27

saleh sargolzaee

614 bronze badges

6 Comments

Dennis Earlyman Over a year ago

Thank you, I'll work with vector now.

saleh sargolzaee Over a year ago

You're welcome. You can also add elements to the vector on the fly : ` for(int i=0; i<str.size(); i++) v.push_back(str[i]); `

463035818_is_not_an_ai Over a year ago

const is not sufficient. With a constant size there would be same error: godbolt.org/z/3oTzs9aze

saleh sargolzaee Over a year ago

You can't assign a non-constant to a constant value either. You can only assign a number directly to a constant, for example: const int N = 10;

463035818_is_not_an_ai Over a year ago

hm? int x = 42; const int y = x; and const int a = 42; int b = a; are both completely fine. I suppose you actually mean a constant expression that can be evaluated at compile time. const isnt that

|

Collectives™ on Stack Overflow

initializing array with variable

2 Answers 2

Comments

6 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

6 Comments

Your Answer

Sign up or log in

Post as a guest

Related