Best practice to use execvp in C++

At the beginning, I wrote something like this

char* argv[] = { "ls", "-al", ..., (char*)NULL };
execvp("ls", argv);

However, GCC popped up this warning, "C++ forbids converting a string constant to char*."

Then, I changed my code into

const char* argv[] = { "ls", "-al", ..., (char*)NULL };
execvp("ls", argv);

As a result, GCC popped up this error, "invalid conversion from const char** to char* const*."

Then, I changed my code into

const char* argv[] = { "ls", "-al", ..., (char*)NULL };
execvp("ls", (char* const*)argv);

It finally works and is compiled without any warning and error, but I think this is a bit cumbersome, and I cannot find anyone wrote something like this on the Internet.

Is there any better way to use execvp in C++?

Solution

You hit a real problem because we are facing two incompatible constraints:

One from the C++ standard requiring you that you must use const char*:

In C, string literals are of type char[], and can be assigned directly to a (non-const) char*. C++03 allowed it as well (but deprecated it, as literals are const in C++). C++11 no longer allows such assignments without a cast.
The other from the legacy C function prototype that requires an array of (non-const) char*:
```
int execv(const char *path, char *const argv[]);
```

By consequence there must be a const_cast<> somewhere and the only solution I found is to wrap the execvp function.

Here is a complete running C++ demonstration of this solution. The inconvenience is that you have some glue code to write once, but the advantage is that you get a safer and cleaner C++11 code (the final nullptr is checked).

#include <cassert>
#include <unistd.h>

template <std::size_t N>
int execvp(const char* file, const char* const (&argv)[N])
{
  assert((N > 0) && (argv[N - 1] == nullptr));

  return execvp(file, const_cast<char* const*>(argv));
}

int main()
{
  const char* const argv[] = {"-al", nullptr};
  execvp("ls", argv);
}

You can compile this demo with:

g++ -std=c++11 demo.cpp

You can see a similar approach in the CPP Reference example for std::experimental::to_array.