Pass string array to method - c++

Question

I am a beginner to C++, I have used C before but never used C++. This is one of my first programs, and it's supposed to do something really simple, however I'm unable to even pass strings between methods... When I call the method setMode with a string array, the method instance recieves an empty array, and not the one I've sent. Why is this happening?
#include <iostream>
#include <fstream>
#include <string>
#include <sstream>
using namespace std;
#define LED_PATH "sys/class/leds/beaglebone:green:usr"
class LED{
private:
string path;
int number;
public:
LED(int number);
virtual void setMode(string mode[]);
virtual ~LED();
};
LED::LED(int number){
ostringstream fs;
this->number = number;
fs << LED_PATH << number;
this->path = string(fs.str());
cout << this->path << endl;
}
LED::~LED(){
}
void LED::setMode(string mode[]){
//Will use all fields of mode[] in the future
cout << "setMode: mode: " << mode[0].c_str() << endl;
}
int main(int argc, char* argv[]){
LED LEDs[4] = {LED(0), LED(1), LED(2), LED(3)};
string mode[argc-1];
//TODO Perform argc value safety check
for(int i=1; i<argc; i++){
mode[i] = string(argv[i]);
cout << mode[i].c_str()<<endl;
}
for(int i=0; i<4; i++){
LEDs[i].setMode(mode);
}
return 0;
}
Output:
debian#beaglebone:~/Desktop/LED_Cpp$ ./led on 1
sys/class/leds/beaglebone:green:usr0
sys/class/leds/beaglebone:green:usr1
sys/class/leds/beaglebone:green:usr2
sys/class/leds/beaglebone:green:usr3
on
1
setMode: mode:
setMode: mode:
setMode: mode:
setMode: mode:

Answer 1

string mode[argc-1];
This uses a proprietary GCC extension. In standard C++, a raw array's size must be known at compile time.
You need something like this instead:
if (argc < 4) {
std::cerr << "error\n";
return EXIT_FAILURE;
}
string mode[4];
Or, what would be very much preferable, use std::vector:
std::vector<std::string> mode(argc-1);
for(int i=1; i<argc; i++){
mode[i] = string(argv[i]);
cout << mode[i].c_str()<<endl;
}
This will leave mode[0] empty.
for(int i=0; i<4; i++){
LEDs[i].setMode(mode);
}
The array (or std::vector) passed to setMode will thus always be such that the first element is an empty string.
void LED::setMode(string mode[]){
This is an attempt to pass an array to a function. What happens really is that a pointer to the first element is passed and the size information is lost.
The correct way of passing a raw array including its size information would be to use a reference:
void LED::setMode(string (&mode)[4])
But as I mentioned previously, just use a std::vector and you'll be fine. When you need to modify the vector, pass it via &, else via const&:
void LED::setMode(std::vector<std::string> const& mode)
In either case, inside of the method, you currenly just access the first element:
cout << "setMode: mode: " << mode[0].c_str() << endl;
As we've established before, mode[0] is always empty. That's why nothing is printed.

Answer 2

You are off by one. You are writing to
mode[1]
in
for(int i=1; i<argc; i++){
mode[i] = string(argv[i]);
and you use
mode[0]
for output.