CODEWITHSUNDEEP
c++carraysstaticconstants
gr4viton
gr4viton

Reputation: 139

constant function parameter as a static array size?

I am not completely sure why this isn't working

void foo(const int a=10){
    const int b = 10;
    int c[a];
    int d[b];
}

I thought that I say to the compiler the a is constant - I even tell it that default value is 10 :).

Why is he yelling:

1>sum_floats_txt.cpp(105): error C2057: expected constant expression
1>sum_floats_txt.cpp(105): error C2466: cannot allocate an array of constant size 0
1>sum_floats_txt.cpp(105): error C2133: 'c' : unknown size

I know what it tells me, but I do not know how to do what I want to do: - function with static array inside of size dependable on outside constant The function is counting something and returning the time spended on the execution (doing it in RTOS).

So I want to call this function with different parameters from main. To find out for which parameter it executes the shortest.

Is the problem in the way the static arrays works? It must have a constant size in the time of compilation. So for different static arrays I must define more of them.

Possible solution is to create more functions or more static arrays with different sizes predefined. And call them one after the other, but there are thousands of different sizes.

I do not want to do the non-automatized iteration to get the best parameters! Could arguments from main passed to the function before execution help?

VS2010 + RTX64 2013 (should not be the problem) win7 thanks!

Upvotes: 1

Views: 872

Answers (2)

ScarletAmaranth
ScarletAmaranth

Reputation: 5101

The const int a argument is "read only" argument. It is not necessarily a value that the compiler knows at compile time. You can use a template argument though.

template <int N = 42>
void foo() {
    int myArray[N];
}

C however does allow what you wrote there, C++ doesn't but will in C++14 with "std::dynarray".

Upvotes: 2

Jens Gustedt
Jens Gustedt

Reputation: 78903

Don't mix up const qualified and constant.

const qualified just says that you don't have the right to modify the variable.

In C++ if a const qualified variable is also a compile time constant, the construct that you are using would be allowed. Here it is a parameter to a function, so there is no way that the compiler may know a value that it would substitute at compile time.

In C, things are different. In modern C, that is C since 1999, variable length arrays with such values that are only known during execution are allowed. Unfortunately there are still C compilers that don't conform to C99. AFAIR micorsoft compilers are among these.

Upvotes: 2

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