C++ Expression must have constant value

#include <iomanip>
#include <iostream>
#include <Windows.h>
using namespace std;

template <class T>
void sort(int n, T a){
for(i=0;i<n-1;i++){
for(j=i;j<n;j++){
if(a[i] > a[j]){
temp=a[i];
a[i]=a[j];
a[j]=temp;
}
}
}
}


void main(){
int size;
cout<<" Please input the amount of numbers you would like to sort"<<endl;
cin>>size;
int Amta[size];
for(int i=0; i<size; i++){
cout<<"Please enter the "<<size+1<< "number";
cin>>Amta[i];
}
Sleep(100000);
}


I am trying to get the how many numbers the user would like to input from the user and store it in the variable size.

But when I initialize array Amta[size] I get the following compile errors:

array Amta[size]

Expression must have constant value

and

C2057: expected constant expression" compile error.

new

delete

std::vector

void main

2 Answers
2

You can't enter a non-constant value between the brackets when you declare your array:

int Amta[size];


Since you're getting size from the user, the compiler can't tell ahead of time how much memory it needs for Amta. The easiest thing to do here (especially for an exercise) is to just choose a relatively large value and make that the constant allocation, like:

size

Amta

int Amta[1024];


And then if you want to be careful (and you should) you can check if (size > 1024) and print an error if the user wants a size that's beyond the pre-allocated bounds.

if (size > 1024)

If you want to get fancy, you can define Amta with no pre-set size, like int *Amta; and then you allocate it later with malloc:

Amta

int *Amta;

malloc

Amta = (int *)malloc(sizeof(int) * size);


Then you must also free Amta later, when you're done with it:

Amta

free(Amta);


malloc

ints

new

malloc()

int

std::vector

C++11 doesn't allow variable length arrays. The size must be a constant. C99 does support it so if you need you can use a C99 compliant compiler. Some compilers like GCC and Clang also support VLA as an extension

But if C++ is a must then you can use alloca() (or _alloca on Windows) to allocate memory in stack and mimic the C99 variable length array behavior

alloca()

_alloca

Amta = (int *)alloca(sizeof(int) * size);


This way you don't need to free the memory after going out the scope because it'll be automatically pop out of stack. However you need to be very careful while using this. It's still better to use std::vector in C++ for these purposes

std::vector

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.