Why can I create a static array dynamically in c++?

Question

I was just going through some simple C++ concepts. I like to think that I am aware of the difference between dynamic and static arrays. But when I run the following code:

`

#include <iostream>
using namespace std;
int main()
{
    int size;
    cout<<"enter size: ";
    cin>>size;
    int arr[size];
    cout<<"enter array values: ";
    for(int i=0;i<size;i++)
        cin>>arr[i];
    for(int i=0;i<size;i++)
        cout<<arr[i]<<" ";
    return 0;
    }

`

It does not give me an error. It shouldn't let me create a static array with the size input from the user right?

C++ doesn't support C VLAa (Variable Length Arrays) -- except by specific non-standard compiler extensions (g++ does). The size must be a literal constant known at compile-time, not determined at run-time. — David C. Rankin
– David C. Rankin, Commented Mar 18, 2020 at 20:44
Some compilers accept this code, as an extension to the C++ language. — spectras
– spectras, Commented Mar 18, 2020 at 20:45
Also, while convenient for short example programs, make sure you read Why is “using namespace std;” considered bad practice?. Starting with good habits leaves nothing that will be difficult to break later :) — David C. Rankin
– David C. Rankin, Commented Mar 18, 2020 at 20:52
Related: https://stackoverflow.com/questions/1887097/why-arent-variable-length-arrays-part-of-the-c-standard — drescherjm
– drescherjm, Commented Mar 18, 2020 at 21:01

bitmask · Accepted Answer · 2020-03-18 21:09:18Z

5

You cannot. This is not valid C++.

It compiles because the compiler you use offers extensions beyond by the C++ standard. Your compiler would have warned you about it if you had enabled compiler warnings:

-Wall -Wextra -pedantic

edited Mar 18, 2020 at 21:09

answered Mar 18, 2020 at 20:52

bitmask

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5 Comments

eerorika Over a year ago

Note that enabling just any warnings (such as -Wall or -Wextra) is not sufficient. It is necessary to use -pedantic to enable standard conformance.

bitmask Over a year ago

@eerorika Thanks. Amended.

Ted Lyngmo Over a year ago

... and sometimes that's not even enough. -pedantic-errors seems to help with the last bit.

bitmask Over a year ago

@TedLyngmo -Werror and sorts are always alluring, but if one gets in the habit of visually reading warnings as errors it becomes unnecessary. Further, there are sometimes situations during debugging where you want stuff like unused arguments and friends to be displayed as warnings but to not break the build.

Ted Lyngmo Over a year ago

I got over that and usually just fix it instead of forcing the compilation through. If I don't agree with the analysis, I either remove that particular analysis step completely or for the single case by adding a comment why the code is better as-is than it would be as suggested by the linter or whatever it is that is complaining.

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