c++ - Why I can use array that initiated with 0 size: Char ch[0]; -

this code:

#include<iostream> using namespace std; int main(){     char ch[0];     cin >> ch;     cout << ch;     return 0; } 

input1:

abcdefghijklmnopqrstuvwxyza 

output1:

abcdefghijklmnopqrstuvwxyza 

(working fine, don't know why)

input2:

abcdefghijklmnopqrstuvwxyzab 

output2:

abcdefghijklmnopqrstuvwxyzab_ 

(request input)

input3:

abcdefghijklmnopqrstuvwxyzabc 

output3: (runtime error)

when output2 request input, , put input2, output same output2 (with request input again), , output1 or output2 appear when put input1 or input2 in there

can explain phenomenon? why happens?

an array of size 0 not valid:

if constant-expression (5.19) present, shall integral constant expression , value shall greater zero.

if compiler accepts it, merely non-standard extension. gcc accepts issue diagnostic if add -pedantic option:

warning: iso c++ forbids zero-size array ‘arr’ [-pedantic] 

nonetheless, reading non-standard array of 0 size undoubtedly give undefined behaviour.