Static array of chars with dynamic size

Question

I'm creating static array of chars which size is defined runtime. And I'm not getting compilation errors.
How is this possible?
Here is my example:

void f(const string& val) {
    char valBuf[val.size() + 1]; strcpy(valBuf, val.c_str());
    cout << valBuf << endl;

  }

int main() {
    string str = "aaaa";
    f(str);

    return 0;
}

Some compilers have this extension. If you compile in strict C++ mode, it should produce an error. — juanchopanza, Aug 13 '14 at 14:36
Variable length arrays are a c99 feature [but several compilers include it as an extension](http://stackoverflow.com/questions/21273829/does-int-size-10-yield-a-constant-expression), possibly a duplicate. — Shafik Yaghmour, Aug 13 '14 at 14:37
If you don't need to change the string, you can use `const char *p = val.c_str();` — Neil Kirk, Aug 13 '14 at 14:40
I've recently asked a couple of questions on VLAs: 1. http://stackoverflow.com/q/24981392/1382251. 2. http://stackoverflow.com/q/24989715/1382251. You might find their answers useful.. — barak manos, Aug 13 '14 at 14:42
This is just a dummy example :) in my real code I'm changing array. — Heghine, Aug 13 '14 at 14:42
@Heghine: "Static array"? Where? I don't see any static arrays in the code you posted. Array `valBuf` is automatic (local), not static. If you attempt to make it static, then even an "extended" compiler will certainly complain. Static arrays are always required to have constant (compile-time) size. — AnT stands with Russia, Aug 13 '14 at 18:18

score 4 · Accepted Answer · answered Aug 13 '14 at 14:37

4

VLAs (i.e. variable length arrays) are a feature of C99 which some C++ compilers (GCC, for example) support as an extension.

This is not allowed under standard C++.

answered Aug 13 '14 at 14:37

Robert Allan Hennigan Leahy

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And construction of the std::string from C-string. But C-string supported even without VLA. Isn't it? – Tanuki Aug 13 '14 at 14:58
Why would C strings depend on VLAs? C strings have been in C since long before VLAs. – Robert Allan Hennigan Leahy Aug 13 '14 at 14:59

Static array of chars with dynamic size

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