const array of const, to use its elements on array length definitions or give template parameters value

Question

I need a constant array of constants, which its constants (the elements of the constant array of constants) can be used where only a compile time constant can be used, like array length definitions.

E.g:

int a[ my_const_array_of_const[0] ]; int b[ my_const_array_of_const[1] ]; template<int p> foo() { ... }; foo< my_const_array_of_const[2] >();

I have tried solutions form other answers, but they were not "constant" enough to the compiler not give an error when using them on above situations.

How can I create the "my_const_array_of_const" constant to compile in such situations?

I need it to configure a High-Level Synthesis (HLS) design. For HLS C++ syntax is restricted. No dynamic memory is allowed, hence I need to use static arrays. Besides, all compilation time constants may be used to optimize the hardware accelerator (that is the reason to use template parameters instead of variables).

As far as I know, macros #define can only "define" scalars. But actually I hace tried something like: #define element(n,d) ==(n) ? d : #define my_const_array(i) (i) element(0,1) (i) element(1,2) (i) element(2,5) 0 The problems are: (1) is not very elegant, (2) the lengtht is limited, (3) and it leads to very long expressions — Ojos
– Ojos, Commented Jun 16, 2017 at 16:57
Because I need to configure a configurable number of objects. I want to keep the configuration parameters of these objects on an array, so I can acces to param_a[0] to object 0, param_a[1] to object 1, etc... — Ojos
– Ojos, Commented Jun 16, 2017 at 17:02

songyuanyao · Accepted Answer · 2017-06-16 16:01:45Z

2

You could use constexpr (since C++11), which guarantee that the value of the element of the array could be evaluated at compile time. e.g.

constexpr int my_const_array_of_const[2] {1, 2};

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edited Jun 16, 2017 at 16:01

answered Jun 16, 2017 at 15:55

songyuanyao

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4 Comments

Ojos Over a year ago

Thanks, it may work on C++11. However, I am guessing that my tool for HLS (Vivado) uses an older version of C++. It gives me the following error: ../../../src/main.cpp:8:1: error: 'constexpr' do not name a type. ../../../src/main.cpp:8:1: note: C++0x 'constexpr' is only available with -std=c++0x o -std=gnu++0x

Ojos Over a year ago

Is there another way to do it with earlier vesrions of C++?

songyuanyao Over a year ago

@Ojos AFAIK, no. Could you compile with the option -std=c++0x？

Ojos Over a year ago

The simulation (i.e. the program compiled and execute on the PC) runs fine with that compiler option (i.e. with the C++11 features). However, the synthesis cannot use the C++ features. If there is no other way to do it, I will need to wait until the synthesis tool implement the new C++11 features

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