CODEWITHSUNDEEP
c++gcc
Agrudge Amicus
Agrudge Amicus

Reputation: 1103

VLA in C++ with std=c++11 extension

How is VLA working in C++ despite giving -std=c++11 compile option. As per GCC:

Variable-length automatic arrays are allowed in ISO C99, and as an extension GCC accepts them in C90 mode and in C++.

#include<iostream>

using namespace std;

int fun()
{
    int j;
    cin>>j;
    return j;
}

int main()
{
    const int i=fun();

    int j[i];
    return 0;
}

Upvotes: 1

Views: 489

Answers (1)

kabanus
kabanus

Reputation: 25980

As Jonathan mentions in a comment, gcc has the pedantic option that also disallows extensions:

-Wpedantic

-pedantic

Issue all the warnings demanded by strict ISO C and ISO C++; reject all programs that use forbidden extensions, and some other programs that do not follow ISO C and ISO C++. For ISO C, follows the version of the ISO C standard specified by any -std option used.

Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few require -ansi or a -std option specifying the required version of ISO C). However, without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option, they are rejected.

-Wpedantic does not cause warning messages for use of the alternate keywords whose names begin and end with ‘__’. This alternate format can also be used to disable warnings for non-ISO ‘__intN’ types, i.e. ‘__intN__’. Pedantic warnings are also disabled in the expression that follows __extension__. However, only system header files should use these escape routes; application programs should avoid them. See Alternate Keywords.

Some users try to use -Wpedantic to check programs for strict ISO C conformance. They soon find that it does not do quite what they want: it finds some non-ISO practices, but not all—only those for which ISO C requires a diagnostic, and some others for which diagnostics have been added.

A feature to report any failure to conform to ISO C might be useful in some instances, but would require considerable additional work and would be quite different from -Wpedantic. We don’t have plans to support such a feature in the near future.

Where the standard specified with -std represents a GNU extended dialect of C, such as ‘gnu90’ or ‘gnu99’, there is a corresponding base standard, the version of ISO C on which the GNU extended dialect is based. Warnings from -Wpedantic are given where they are required by the base standard. (It does not make sense for such warnings to be given only for features not in the specified GNU C dialect, since by definition the GNU dialects of C include all features the compiler supports with the given option, and there would be nothing to warn about.)

-pedantic-errors

Give an error whenever the base standard (see -Wpedantic) requires a diagnostic, in some cases where there is undefined behavior at compile-time and in some other cases that do not prevent compilation of programs that are valid according to the standard. This is not equivalent to -Werror=pedantic, since there are errors enabled by this option and not enabled by the latter and vice versa.

The emboldened part is my emphasis — apparently some things may still slip by. Still, I find this option enough for my needs.

Upvotes: 3

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