Sudden problem with std::vector

Code:

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uint32_t some_func() { return 32; }

int buf_1[32];          // <--- this always compiled with MSVC
int buf_2[some_func()]; // <--- whereas this fails because the buffer size needs to be known at compile time

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In the past I always got around this by using std::vector - i.e.

Code:

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uint32_t some_func() { return 32; }

const int x = some_func(); // <--- I'm sure these lines
std::vector<int> buf_3[x]; // <--- previously compiled

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But all of a sudden this morning, the std::vector line is giving me C2131 because the value of x isn't known at compile time. I'm probably missing something obvious but I'm sure this used to compile okay... can anyone see what I'm doing wrong? :confused:

Oops - I was using square brackets instead of round :blush:

This works...

Code:

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std::vector<int> buf_3(x);

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Quote:

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Originally Posted by John E

Code:

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int buf_2[some_func()]; // <--- whereas this fails because the buffer size needs to be known at compile time

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It will work (in this case) if the function is declared constexpr.

Thanks for the tip wolle. I'd never even heard of constexpr but you're right - it works fine!

Quote:

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Originally Posted by John E

it works fine!

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But, it only works if the function itself is executable at compile-time. If not, you still need to use an std::vector.

Ah yes, you're right - in fact even something as simple as this doesn't work...

Code:

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constexpr uint32_t some_func() { return sqrt(1024); }

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For a constexpr function to return a constexpr value, any functions it calls must also be marked as constexpr. ie. can be evaluated at compile time. Unfortunately, sqrt() isn't one of them.

See https://en.cppreference.com/w/cpp/language/constexpr

and the example of a compile-time factorial function.