Fill a char array with int values [closed] - c++

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I'm using a library for an Arduino project that has a function I need to call. This function accepts just one parameter of type const char*. Let's call it foo.
I need to pass some int values to foo, so I convert them first using sprintf. So far so good.
The problem comes when I try to fill an array with int values converted to char and then call foo with each of the values in the array.
I hope this explains the problem better:
#include <iostream>
using namespace std;
// This function cannot be modified because
// is a part of a library
void foo(const char *bar){
cout << "Result: " << bar << endl;
}
int main() {
char *values[10]; // My array of values
char tmp[10]; // Temporary buffer for the int > char conversion
for(int i = 0; i < 10; i++){
int samplevalue = i * 2; // Just a sample value, not important
sprintf(tmp, "%d", samplevalue); // Copy the sample value to the temporary buffer
values[i] = tmp; // Assign the value of the temp var to a position in my values array
cout << values[i] << endl;
}
cout << "==============" << endl;
// Here comes the problem:
for(int i = 0; i < 10; i++){
foo(values[i]);
}
return 0;
}
The output of that code is the following:
0
2
4
6
8
10
12
14
16
18
==============
Result: 18
Result: 18
Result: 18
Result: 18
Result: 18
Result: 18
Result: 18
Result: 18
Result: 18
Result: 18
As you can see, all of the Result lines are equal to the last value assigned to the tmp var. I guess it's because each of the values in the values[10] array contains a pointer to the tmp variable instead of its actual value.
What I'd like to have is a different number on each Result line, as in the first for loop.
I guess it's pretty obvious that I'm not even near to be a C++ expert and any help will be highly appreciated.
Thanks!

Answer 1

A char * pointer and an array are not a string. Use std::string instead.
#include <iostream>
using namespace std;
// This function cannot be modified because
// is a part of a library
void foo(const char *bar)
{
cout << "Result: " << bar << endl;
}
int main(void)
{
std::string values[10]; // My array of values
char tmp[10]; // Temporary buffer for the int > char conversion
for (int i = 0; i < 10; i++) {
int samplevalue = i * 2; // Just a sample value, not important
sprintf(tmp, "%d", samplevalue); // Copy the sample value to the temporary buffer
values[i] = tmp; // Assign the value of the temp var to a position in my values array
cout << values[i] << endl;
}
cout << "==============" << endl;
// Here comes the problem:
for (int i = 0; i < 10; i++) {
foo(values[i].c_str());
}
return 0;
}
When using an array, all the pointers in your values array point to tmp, you can check that by looping through values and printing the address like this
fprintf(stdout, "%p\n", values[i]);
So because you sprintf() into tmp all the values, the value that will be printed is always the last one, there is no copy implied in
values[i] = tmp;
that just makes values[i] point to tmp, so when you access values[i] you really access tmp.
With std::string copy occurs.
Also, you should probably use a string stream to write numbers into each values[i] directly, because sprintf() is very dangerous.
Or even better use a real c++ solution like this one,
#include <iostream>
#include <vector>
#include <sstream>
// This function cannot be modified because
// is a part of a library
void foo(const char *bar)
{
std::cout << "Result: " << bar << std::endl;
}
int main(void)
{
std::vector<std::string> values;
for (int i = 0; i < 10; i++) {
values.push_back(std::to_string(2 * i));
std::cout << values[i] << std::endl;
}
std::cout << "==============" << std::endl;
for (size_t i = 0; i < values.size(); i++) {
foo(values[i].c_str());
}
return 0;
}
Note that now, you can change the number of elements in values and you can use it as an array if you need to, just read the documentation for std::vector.

Answer 2

Ok, I finally got to get it working. In Arduino strings are declared as String variable;, and the c_str() function converts a string into a const char *, so I convert the int number to String, and then to const char *:
for(int i = 0; i < 10; i++){
String tmp = String(i * 2);
values[i] = tmp.c_str();
}
And that's it! It works now :)