How can I pass a constant int via command line argument in C++?

Question

I want to be able to pass an integer via command line argument that I will call length. I need to make this constant, because it will be used to determine the size of several bitsets. I have tried doing this many ways, for example:

int main(int arc, const char* argv[]){
        const int * ptr;
        if (!(istringstream{argv[1]} >> ptr)) { return 1;}
        const int length = *ptr;

        bitset<length> right_ones = 1;
        return 0;
}

I also tried it this way:

int main(int arc, const char* argv[]){
        int test_int;   
        if (!(istringstream{argv[1]} >> test_int)) { return 1;}
        const int length = argv[1];

        bitset<length> right_ones = 1;
        return 0;
}

But none of the ways I have tried it worked. How can I accomplish this?

If it's passed via a command line argument, it is obviously not constant. — harold, May 09 '17 at 17:15
Get your DeLorean up to 88 mph. Run the program, observe the user's future input, then go back and add it. — molbdnilo, May 09 '17 at 17:19
Templates can only be used with compile time constant. Values passing before running are obviously not compile time constants — phuclv, May 09 '17 at 17:19
If this is meant to be a compile time constant, why don't you make it a compiler argument (like in a qmake project file, depending what you use) instead of a run time argument? — Aziuth, May 09 '17 at 17:22
A template argument has to be compile-constant and hence can not be derived directly from a runtime value (cmdline argument). If you insist, you could use a switch and instantiate for several compile-time values — André, May 09 '17 at 17:25
Unless it changes with C++17, you can use `std::vector`. In ways, it behaves more like a `bitset` than a `vector`, so I recall an intent to change it and introduce a new class to replace the functionality. — Weak to Enuma Elish, May 09 '17 at 17:34
@JamesRoot correct [Why is vector not a STL container?](http://stackoverflow.com/q/17794569/995714) — phuclv, May 11 '17 at 10:34

score 4 · Accepted Answer · edited May 23 '17 at 12:03

4

If it is going to be passed as a command line argument, then you cannot make it a constant.

By the way, this is relevant to your case: Define bitset size at initialization?

edited May 23 '17 at 12:03

Community

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answered May 09 '17 at 17:18

gsamaras

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Okay, I think I misunderstood what a const was...Is there anyway to use bitset with length not being constant then? As long as I can pass the size of the bitset via command line my problem will be solved. – partyphysics May 09 '17 at 17:22
I updated my answer @partyphysics. Read the link for more! – gsamaras May 09 '17 at 17:23
Thanks, the boost::dynamic_bitset solves the problem – partyphysics May 16 '17 at 16:18

score 2 · Answer 2 · answered May 09 '17 at 17:21

2

The length of the bitset is needed at compile time so no matter how you try it there is no way to pass that value at runtime.

answered May 09 '17 at 17:21

grigor

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How can I pass a constant int via command line argument in C++?

2 Answers2