array bound is not an integer constant before ']' token

Question

main:

#include <iostream>
#include "common.h"
#include "squares.h"
#include "board.h"
using namespace std;
int Board::board_length=8;
int main()
{
    Board *tabla=new Board();
    tabla->printBoard();
}

board.h :

#ifndef BOARD_H
#define BOARD_H
#include "squares.h"

class Board
{
   static int board_length;
   Square boardSquares[board_length][board_length];
   public:
   Board();
   void printBoard();

};

error line 8 in board.h C:\c++ projects\CHESS-P3\board.h|8|error: array bound is not an integer constant before ']' token|

The error is in `board.h` which you don't show us. Please always show us a proper [mcve] and copy-paste the full and complete build output. Please [edit] your question. — Some programmer dude, Mar 10 '21 at 16:13
As a ***guess***, you use a variable or other non-constant value as the size of an array. In C++ all array sizes must be compile-time constants. — Some programmer dude, Mar 10 '21 at 16:14
are you using `int Board::board_length=8;` as size of an array? Thats not a constant. — 463035818_is_not_an_ai, Mar 10 '21 at 16:14
You're probably running afoul of the [variable-length array](https://stackoverflow.com/questions/1887097/why-arent-variable-length-arrays-part-of-the-c-standard) issue in the code you haven't shown. — Nathan Pierson, Mar 10 '21 at 16:14
By the way, there's no reason to use a pointer in this code snippet: `Board tabla; tabla.printBoard();`—leak gone. — chris, Mar 10 '21 at 16:20
Yep, the commenters guessed right. You are using variable length array. One thing you can do (if you are not using ancient compiler) is to change `board_length` to `constexpr static int board_length = 8;` (and remove definition from `main.cpp`). This way you will have a compile time constant, which can be used to create an array. https://wandbox.org/permlink/9v8Cn7VyeE1ploEx# — Yksisarvinen, Mar 10 '21 at 16:22
@chris if i don t use the pointer i have another error: conversion from board* to non-scalar type Board — palalele, Mar 10 '21 at 16:23
@palalele, Don't use `new` if you can avoid it. The code I showed does the same thing but without the memory leak — chris, Mar 10 '21 at 16:24
@palalele If you look at the suggested code from chris then you will see that it doesn't use `new`. — Some programmer dude, Mar 10 '21 at 16:26
@Yksisarvinen thanks, it s working now. sorry for my mistake guys — palalele, Mar 10 '21 at 16:26

Striezel · Accepted Answer · 2021-03-10T16:50:55.137

As the error message indicates, board_length is not a constant value. To fix that, change the line

static int board_length;

in board.h to

static const int board_length = 8;

and remove the line

int Board::board_length=8;

from your main file. This should compile, but I cannot tell for sure, since you did not provide a minimal, reproducible example.

Bonus: To avoid a memory leak you should also change

Board *tabla=new Board();
tabla->printBoard();

in main just to

Board tabla;
tabla.printBoard();

Since you do not seem to be passing the Board instance around, there is no need to use a pointer here.

As a general rule of thumb: Whenever there is a new somewhere, there also needs to be a corresponding delete. Otherwise your pogram may leak memory. While this is no big deal for a small, short-running example program, it can become a serious issue for a program that runs for a long time.

Serge Ballesta · Answer 2 · 2021-03-10T16:56:08.357

This is a nice use case for an int template:

board.h

template<int _board_length>
class Board
{
   static const int board_length = _board_length;
   int boardSquares[_board_length][_board_length];
   public:
   Board(){};
   void printBoard(){};

};

main.cpp

...
constexpr int BL=8;

//odr declare the static member if it matters
template<>
const int Board<BL>::board_length;


int main() {
    Board<BL> tabla;
    tabla.printBoard();
}

That way, the actual dimensions of the array are defined in the main and not in the include file where the class is declared, and all the methods in Board can know the actual size.

array bound is not an integer constant before ']' token

2 Answers2