Defining a constant by a variable in C++

Question

So I need to define a constant by a variable so I can use that value in the definition of an array. Will this work?

std::fstream scores("scores.txt");
int numberOfLines;
std::string temp;

while (std::getline(scores, temp))
{
    numberOfLines++;
}

const int numberOfLines1 = numberOfLines;
int scoresArr [numberOfLines1] = {};
scores.close();

I think you are asking something similar to [Does “int size = 10;” yield a constant expression?](https://stackoverflow.com/q/21273829/1708801) — Shafik Yaghmour, Nov 03 '18 at 22:56
The important part of the requirement is *compile-time*, not *constant* — Caleth, Nov 05 '18 at 14:40

max66 · Answer 1 · 2018-11-05T14:07:18.610

Will this work?

Short answer: no.

Long answer.

A C-style array need (in standard C++) a compile-time-known size.

So

// ............vvvvvvvvvvvvvv  <-- compile time constant, please
int scoresArr [numberOfLines1] = {};

numberOfLines1 must be known compile-time.

Unfortunately, in your code, numberOfLines1's value isn't known compile time but it's clearly uninitialized (so starts with an undefined value)

int numberOfLines;  // <-- initialized with an undefined value

and incremented a number of times that depends from an external file, so incremented necessarily run-time

while (std::getline(scores, temp))
{
    numberOfLines++;
}

So: no, doesn't works.

score 0 · Answer 2 · answered Nov 04 '18 at 00:44

0

You could use int *scoresArr = new int[numberOfLines1];

and free the space afterwards with delete [] scoresArr

answered Nov 04 '18 at 00:44

wiomoc

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Defining a constant by a variable in C++

2 Answers2