Question

I have a class with parse(int argc, char* argv[]) function which I have to use to set a desired state of an object. I’m taking the parameters from the gui using stringstream and then I’m trying to convert them to char** to pass them to the function. Here’s what I’ve got:

std::stringstream sstream;

sstream << "-clip" << " " << min_x_entry.get_text()
        << " " << max_x_entry.get_text(); // etc.

std::cout << sstream.str();    // All looks good here

std::vector<std::string> args;
std::vector<char*> argv;
std::string arg;

while (sstream >> arg)
{
    args.push_back(arg);
    argv.push_back(const_cast<char*>(args.back().c_str()));
}
argv.push_back(0);

int argc = args.size();

for (int i = 0; i < argc; ++i)
    std::cout << &argv[0][i];    // This outputs garbage

my_object.parse(argc, &argv[0])  // And this fails

What am I missing? Is there a better way of achieving this?

Answer 1

A problem would be reallocation of the args vector as push_back() will grow the size of the vector if required:

If new size() is not larger than capacity(), no iterators or references are invalidated. Otherwise all iterators and references are invalidated.

The argv vector is storing pointers to the internals of elements in args, so these would be invalidated.

A solution would be to create the args vector first then create the argv vector afterwards:

while (sstream >> arg) args.push_back(arg);

for (auto i = args.begin(); i != args.end(); i++)
{
    argv.push_back(const_cast<char*>(i->c_str()));
}
argv.push_back(0);

The for loop that prints out the argv strings is incorrect. This:

&argv[0][i]

is a char* but starts from ith element of the first entry in argv. For example, if the first c-string in argv was "string":

&argv[0][1] is "tring"
&argv[0][2] is "ring"

change to:

for (int i = 0; i < argc; i++)
    std::cout << argv[i] << std::endl; // Added 'endl' to flush 'cout'.