Dynamic allocation of array

Question

Why the below code compilation success though dynamic allocation of array is not possible in C++? on uncommenting the comments it shows error??

#include<iostream> #include <string> using namespace std; int main() { string aa; cin>>aa; int a[aa.size()];// though allocating the array dynamically the compilation succeeded cout<<"COMPILATION SUCCESS"<<endl; /*char *p; cin>>p; int y=sizeof(p); int b[y]; cout<<"COMPILATION ERROR"<<endl; */ /* int tt; cin>>tt; int c[tt];// shows error cout<<"ERROR"; */ }

Are you sure it's cin<<tt; instead of cin>>tt;?

johnchen902
– johnchen902

2013-06-27 13:19:02 +00:00
Commented Jun 27, 2013 at 13:19 — johnchen902
– johnchen902, Commented Jun 27, 2013 at 13:19
Arrays must have a size that is known at compile time.

Kerrek SB
– Kerrek SB

2013-06-27 13:20:19 +00:00
Commented Jun 27, 2013 at 13:20 — Kerrek SB
– Kerrek SB, Commented Jun 27, 2013 at 13:20

user529758 · Accepted Answer · 2013-06-27 13:17:16Z

Because you appear to be using a compiler which allows this. VLAs in C++ are a GNU extension, any chance you are compiling this with g++ or clang++?

Set your compiler to strict ISO C++ mode, and it will warn you or error out.

What I get from clang++:

h2co3-macbook:~ h2co3$ clang++ -o quirk quirk.cpp -Wall -std=c++11 -pedantic quirk.cpp:6:9: warning: variable length arrays are a C99 feature [-pedantic,-Wvla] char cs[s.size() + 1]; ^ quirk.cpp:6:7: warning: unused variable 'cs' [-Wunused-variable] char cs[s.size() + 1]; ^ 2 warnings generated.

@Fanael: In C++14, both VLA's and dynarray (an alternative), are both options being considered, but I would refrain from saying "They're standard" until the standard actually comes out.

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