Inserting double into char* array element - c++

Question

How do I insert a double data type into a char* array? I've tried using std::to_string() as well as c_str() however neither seem to be doing the trick.
Essentially I am trying to call a c++ program using execvp() which from reading the man page I think it needs to take in a char* array.
I am trying to call the program, then pass the doubles a, b, and c as the argv parameters for the program execvp() is calling.
E.G. when a = 0.1, b =0.1, c = 0.1 execvp(args[0], args) should do the same as the terminal call: ./RL 0.1 0.1 0.1
#include <iostream>
#include <unistd.h>
#include <stdlib.h>
#include <string>
int main()
{
char *args[5];
args[0] = "RL";
std::string temp;
for(double a = 0.1; a < 1; a += 0.1){
for(double b = 0.1; b < 1; b += 0.1){
for(double c = 0.1; c < 1; c+= 0.1){
temp = std::to_string(a); //attempted to convert the double to a string
args[1] = temp.c_str(); //attempted to convert the string to c string
temp = std::to_string(b);
args[2] = temp; //attempted to just insert the string
args[3] = c; //attempted to insert as double
std::cout << "Calling: " << a << " " << b << " " << c << std::endl;
execvp(args[0], args);
}
}
}
return 0;
}
I get errors like this:
RLdriver.cpp:14:32: error: assigning to 'char *' from incompatible type 'const
std::__1::basic_string<char, std::__1::char_traits<char>,
std::__1::allocator<char> >::value_type *' (aka 'const char *')
args[1] = temp.c_str();
~~~~~^~~~~~~
RLdriver.cpp:16:27: error: assigning to 'char *' from incompatible type
'std::string' (aka 'basic_string<char, char_traits<char>, allocator<char>
>')
args[2] = temp;
^~~~
RLdriver.cpp:17:27: error: assigning to 'char *' from incompatible type 'double'
args[3] = c;
EDIT: I'm aware exec() would not really work in this sense of looping as it replaces the current program, but I know how to deal with that later on, just trying to get the passing of args working.

Answer 1

There are two problems with this code:
you cannot fill your "array of char pointers" args with temporary objects as you try, this will not reflect what you need. Use instead (see below) const char* const args[] and fill it with pointers to real object data tempa, tempb, tempc.
execvp expects char* const[] while in C++ you would always produce const char* const[], so you need to cast something like execvp(const_cast<char* const*>(args));
You may change your code to, then it will work:
#include <iostream>
#include <unistd.h>
#include <stdlib.h>
#include <string>
int main()
{
for(double a = 0.1; a < 1; a += 0.1){
for(double b = 0.1; b < 1; b += 0.1){
for(double c = 0.1; c < 1; c+= 0.1){
std::string tempa = std::to_string(a);
std::string tempb = std::to_string(b);
std::string tempc = std::to_string(c);
const char* const args[] = {tempa.c_str(), tempb.c_str(), tempc.c_str(), nullptr};
std::cout << "Calling: " << a << " " << b << " " << c << std::endl;
execvp(args[0], const_cast<char*const*>(args));
}
}
}
return 0;
}
(tested)

Answer 2

Use of
args[1] = temp.c_str();
is a problems since the LHS is of type char* and the RHS is of type char const*. You can't assign a char const* to a char*.
A simple fix is to use:
args[1] = strdup(temp.c_str());
strdup is not a standard library function but some compilers support it natively. If your compiler does not support it, it's not too hard to implement it yourself. Lookup the on the web and you fill most likely find one.
Make sure that you deallocate the string returned by strdup. You'll have to consult your compiler's documentation to figure out how memory was allocated in strdup. You'll need to use the appropriate deallocation mechanism.

Answer 3

Your first way, i.e.
temp = std::to_string(a); //attempted to convert the double to a string
args[1] = temp.c_str(); //attempted to convert the string to c string
is correct, except that you need to change the declaration char *args[5]; to const char *args[5];. Of course, if you want to pass 3 different command line arguments, you need 3 different temp variables (temp1, temp2, temp3).