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POC++ about dynamic array on stack
 

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    <p>So far as I know,C++/C doen't support dynamic array on stack . In the following delcaration :</p> <pre><code>int data[n] ; // if the n is not decided at compiling time ,this leads to error </code></pre> <p>But recently , I read some other guys code as following:</p> <pre><code>//** It seems the n number can not be decided at compling time,but when I run it , if i fprintf the formation, each time i got the correct array size !!!!!! the G++ version is 4.7.1 Is this because the G++ 4.7.1 support C++11 x which allow dynamic array? **// #include &lt;cstdio&gt; #include &lt;algorithm&gt; using namespace std; #include &lt;stdio.h&gt; char s[31]; int Hash() { int sum=0; for(int i=0,k=0;k&lt;7;i++) { if(s[i]&gt;='0'&amp;&amp;s[i]&lt;='9') { sum*=10;k++; sum+=(s[i]-'0'); } else if(s[i]&gt;='A'&amp;&amp;s[i]&lt;'Z') { sum*=10;k++; sum+=((s[i]-'A'-(s[i]&gt;'Q'))/3+2); } } return sum; } int main() { int n;scanf("%d",&amp;n); int data[n];getchar(); //fprintf(stderr,"size is %d\n",sizeof(data)/sizeof(data[0])); //** It seems the n number can not be decided at compling time,but when I run it , if i fprintf the formation, each time i got the correct array size !!!!!! *// for(int tmp=0;tmp&lt;n;tmp++) { gets(s); data[tmp]=Hash(); } sort(data,data+n); bool p=false;n--; for(int i=0,num=1;i&lt;n;i+=num=1) { while(data[i]==data[i+1]) { num++; i++; } if(num&gt;1) { printf("%03d-%04d %d\n",data[i]/10000,data[i]%10000,num); p=true; } } if(!p)printf("No duplicates.\n"); return 0; } </code></pre>
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    1. COThe confusion usually comes from C++11 §1.1: "C++ is [...] based on the C programming language as specified in ISO/IEC 9899:1999." C99 does support VLAs, but C++11 does not (use gcc's `--pedantic` or `--std=c++11`). **Related**: http://stackoverflow.com/questions/8593643/does-c-support-variable-length-arrays
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    2. COIn Standards-compliance C++, using a `vector` is typical in this situation. If you *must* use the stack, on many platforms you could use `alloca()` but you'll need to use placement-`new` to initialise the elements then invoke the destructors manually. It's fiddly and error-prone e.g. ensuring destruction of the right set of elements after an exception (that might have been thrown during one of those placement-`new`s)
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