C++: Built-in arrays












-2














I thought that built-in arrays in C++ are statically allocated. But the following code works:



//...

int x;

std::cin >> x;

const int cx = x;

int array[cx];

//...


Why does it?






c++







share|improve this question




















  • 8




    That code works on some compilers that offer default-on extensions for variable length array. It doesn't work on other compilers and is not portable c++. You're being tricked by your compiler into thinking your code is valid c++. Another example of why you can't learn c++ by trial and error.
    – François Andrieux
    Nov 20 '18 at 19:18












  • One reason Variable Length Arrays are bad: x can support numbers that are very big. Bigger than the amount of Automatic storage available to that array. Now multiply that large number by the size of an int. Using an array of that size will commonly manifest as a stack overflow and much debugging can ensue as a result.
    – user4581301
    Nov 20 '18 at 19:58
















-2














I thought that built-in arrays in C++ are statically allocated. But the following code works:



//...

int x;

std::cin >> x;

const int cx = x;

int array[cx];

//...


Why does it?






c++







share|improve this question




















  • 8




    That code works on some compilers that offer default-on extensions for variable length array. It doesn't work on other compilers and is not portable c++. You're being tricked by your compiler into thinking your code is valid c++. Another example of why you can't learn c++ by trial and error.
    – François Andrieux
    Nov 20 '18 at 19:18












  • One reason Variable Length Arrays are bad: x can support numbers that are very big. Bigger than the amount of Automatic storage available to that array. Now multiply that large number by the size of an int. Using an array of that size will commonly manifest as a stack overflow and much debugging can ensue as a result.
    – user4581301
    Nov 20 '18 at 19:58














-2












-2








-2







I thought that built-in arrays in C++ are statically allocated. But the following code works:



//...

int x;

std::cin >> x;

const int cx = x;

int array[cx];

//...


Why does it?






c++







share|improve this question















I thought that built-in arrays in C++ are statically allocated. But the following code works:



//...

int x;

std::cin >> x;

const int cx = x;

int array[cx];

//...


Why does it?






c++




c++






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited Nov 20 '18 at 19:49









Fei Xiang

2,0614621




2,0614621










asked Nov 20 '18 at 19:17









Glech

6




6








  • 8




    That code works on some compilers that offer default-on extensions for variable length array. It doesn't work on other compilers and is not portable c++. You're being tricked by your compiler into thinking your code is valid c++. Another example of why you can't learn c++ by trial and error.
    – François Andrieux
    Nov 20 '18 at 19:18












  • One reason Variable Length Arrays are bad: x can support numbers that are very big. Bigger than the amount of Automatic storage available to that array. Now multiply that large number by the size of an int. Using an array of that size will commonly manifest as a stack overflow and much debugging can ensue as a result.
    – user4581301
    Nov 20 '18 at 19:58














  • 8




    That code works on some compilers that offer default-on extensions for variable length array. It doesn't work on other compilers and is not portable c++. You're being tricked by your compiler into thinking your code is valid c++. Another example of why you can't learn c++ by trial and error.
    – François Andrieux
    Nov 20 '18 at 19:18












  • One reason Variable Length Arrays are bad: x can support numbers that are very big. Bigger than the amount of Automatic storage available to that array. Now multiply that large number by the size of an int. Using an array of that size will commonly manifest as a stack overflow and much debugging can ensue as a result.
    – user4581301
    Nov 20 '18 at 19:58








8




8




That code works on some compilers that offer default-on extensions for variable length array. It doesn't work on other compilers and is not portable c++. You're being tricked by your compiler into thinking your code is valid c++. Another example of why you can't learn c++ by trial and error.
– François Andrieux
Nov 20 '18 at 19:18






That code works on some compilers that offer default-on extensions for variable length array. It doesn't work on other compilers and is not portable c++. You're being tricked by your compiler into thinking your code is valid c++. Another example of why you can't learn c++ by trial and error.
– François Andrieux
Nov 20 '18 at 19:18














One reason Variable Length Arrays are bad: x can support numbers that are very big. Bigger than the amount of Automatic storage available to that array. Now multiply that large number by the size of an int. Using an array of that size will commonly manifest as a stack overflow and much debugging can ensue as a result.
– user4581301
Nov 20 '18 at 19:58




One reason Variable Length Arrays are bad: x can support numbers that are very big. Bigger than the amount of Automatic storage available to that array. Now multiply that large number by the size of an int. Using an array of that size will commonly manifest as a stack overflow and much debugging can ensue as a result.
– user4581301
Nov 20 '18 at 19:58












1 Answer
1






active

oldest

votes


















3














Variable length arrays are not part of the C++ standard, however your compiler allows it. If you use the -pedantic-errors option with your compiler (assuming g++), this will throw an error as that option strictly enforces the standard.






share|improve this answer



















  • 1




    -pedantic causes the compiler to emit warnings, not errors. You need to add on -Werror or -pedantic-errors to get an compilation-halting error. That said, -Werror is something I recommend to anyone learning C++. Warnings are the first line of defense against logic errors and you should not ignore them.
    – user4581301
    Nov 20 '18 at 19:47












  • thanks, edited my answer to reflect that
    – jdrd
    Nov 20 '18 at 19:51











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1 Answer
1






active

oldest

votes








1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes









3














Variable length arrays are not part of the C++ standard, however your compiler allows it. If you use the -pedantic-errors option with your compiler (assuming g++), this will throw an error as that option strictly enforces the standard.






share|improve this answer



















  • 1




    -pedantic causes the compiler to emit warnings, not errors. You need to add on -Werror or -pedantic-errors to get an compilation-halting error. That said, -Werror is something I recommend to anyone learning C++. Warnings are the first line of defense against logic errors and you should not ignore them.
    – user4581301
    Nov 20 '18 at 19:47












  • thanks, edited my answer to reflect that
    – jdrd
    Nov 20 '18 at 19:51
















3














Variable length arrays are not part of the C++ standard, however your compiler allows it. If you use the -pedantic-errors option with your compiler (assuming g++), this will throw an error as that option strictly enforces the standard.






share|improve this answer



















  • 1




    -pedantic causes the compiler to emit warnings, not errors. You need to add on -Werror or -pedantic-errors to get an compilation-halting error. That said, -Werror is something I recommend to anyone learning C++. Warnings are the first line of defense against logic errors and you should not ignore them.
    – user4581301
    Nov 20 '18 at 19:47












  • thanks, edited my answer to reflect that
    – jdrd
    Nov 20 '18 at 19:51














3












3








3






Variable length arrays are not part of the C++ standard, however your compiler allows it. If you use the -pedantic-errors option with your compiler (assuming g++), this will throw an error as that option strictly enforces the standard.






share|improve this answer














Variable length arrays are not part of the C++ standard, however your compiler allows it. If you use the -pedantic-errors option with your compiler (assuming g++), this will throw an error as that option strictly enforces the standard.







share|improve this answer














share|improve this answer



share|improve this answer








edited Nov 20 '18 at 19:50

























answered Nov 20 '18 at 19:40









jdrd

1297




1297








  • 1




    -pedantic causes the compiler to emit warnings, not errors. You need to add on -Werror or -pedantic-errors to get an compilation-halting error. That said, -Werror is something I recommend to anyone learning C++. Warnings are the first line of defense against logic errors and you should not ignore them.
    – user4581301
    Nov 20 '18 at 19:47












  • thanks, edited my answer to reflect that
    – jdrd
    Nov 20 '18 at 19:51














  • 1




    -pedantic causes the compiler to emit warnings, not errors. You need to add on -Werror or -pedantic-errors to get an compilation-halting error. That said, -Werror is something I recommend to anyone learning C++. Warnings are the first line of defense against logic errors and you should not ignore them.
    – user4581301
    Nov 20 '18 at 19:47












  • thanks, edited my answer to reflect that
    – jdrd
    Nov 20 '18 at 19:51








1




1




-pedantic causes the compiler to emit warnings, not errors. You need to add on -Werror or -pedantic-errors to get an compilation-halting error. That said, -Werror is something I recommend to anyone learning C++. Warnings are the first line of defense against logic errors and you should not ignore them.
– user4581301
Nov 20 '18 at 19:47






-pedantic causes the compiler to emit warnings, not errors. You need to add on -Werror or -pedantic-errors to get an compilation-halting error. That said, -Werror is something I recommend to anyone learning C++. Warnings are the first line of defense against logic errors and you should not ignore them.
– user4581301
Nov 20 '18 at 19:47














thanks, edited my answer to reflect that
– jdrd
Nov 20 '18 at 19:51




thanks, edited my answer to reflect that
– jdrd
Nov 20 '18 at 19:51


















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