Is there a way to avoid array-to-pointer decay?

Question

The parameter to main char* argv[] decays to char**, which is unfortunate, because it cannot be used with std::begin which only accepts arrays. The only workaround I see is to use a variable length array which is undesirable.

#include <iostream>

int main(int argc, char* argv[])
{
    char* _argv[argc];
    for (int i = 0; i < argc; ++i)
        _argv[i] = argv[i];

    for (arg : _argv)
    {
        std::cout << arg << " ";
    }
    return 0;
}

Desirably I want something like: char* _argv[] = { ... };

In fact, you don't even have array to pointer decay. argv really is char**. But what problem re you string to solve? — juanchopanza
– juanchopanza, Commented Oct 7, 2014 at 8:43
char* _argv[argc]; is not standard C++, since there are no variable length arrays in C++. — TNA
– TNA, Commented Oct 7, 2014 at 8:54

TonyK · Accepted Answer · 2014-10-07 08:47:20Z

6

That is not possible, since the signature of main is fixed. You can copy the elements to a vector using:

std::vector<std::string> args {argv, argv + argc};

Then you can use std::begin on args

edited Oct 7, 2014 at 8:47

TonyK

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answered Oct 7, 2014 at 8:45

TNA

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Comments

jxh · Accepted Answer · 2014-10-07 08:54:11Z

5

You can define a simple class wrapper to give you begin() and end().

struct Args {
    int argc_;
    char **argv_;
    Args (int argc, char *argv[]) : argc_(argc), argv_(argv) {}
    char ** begin () { return argv_; }
    char ** end () { return argv_ + argc_; }
};

int main(int argc, char *argv[]) {
    for (auto s : Args(argc, argv)) {
        std::cout << s << '\n';
    }
    return 0;
}

answered Oct 7, 2014 at 8:54

jxh

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Comments

Columbo · Accepted Answer · 2014-10-07 08:57:30Z

3

The only workaround I see is to use a variable length array which is undesirable.

The only workaround for what exactly? There is no problem. If you actually need an array of strings, use a vector as shown in TNAs answer; I doubt you need to modify the parameters though.
If you just need to iterate through it, there is no need to copy all the pointers.

for (auto str : boost::make_iterator_range(argv, argv + argc))
    std::cout << str;

Or

for (auto ptr = argv; ptr != argv + argc; ++ptr)
    std::cout << *ptr << '\n';

The parameter to main char* argv[] decays to char**, which is unfortunate, because it cannot be used with std::begin which only accepts arrays.

There is no other way. The amount of command line parameters is not known at compile time, and this method of passing them (instead of using two pointers-to-pointers) is historically motivated.

edited Oct 7, 2014 at 8:57

answered Oct 7, 2014 at 8:50

Columbo

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1 Comment

user4116340 Over a year ago

Thank you for a solution that has no memory allocations.

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