Vector in c++ using for loop

Question

I am new to c++ in general. So I have been trying to learn about using vectors after someone recently helped with using an Arduino type project to read RFID tags. It really got me thinking I have no clue how to program. So I hit the books!

So here is the question: When I do the following code:

#include <iostream>
#include <vector>

struct Runner{
    char runnerTag[32];
    uint32_t ts;
};

std::vector<Runner > runners;

int main() {
    std::cout << "Hello, Runners!\n";
    for (int i = 0; i < 100; i++) {
        std::string runnertg = "testTrackTag01";
        uint32_t timeStamp = rand() % 100 + 1;
        runners[i] = new Runner({runnertg, timeStamp});
    }
    return 0;
}

I get this annoying little message from xcode:

No matching constructor for initialization of 'Runner'

on line 16 of the above snippet. What in the world am I doing wrong?

Answer 1

1. The expression new Runner({runnertg, timeStamp}) has a type mismatch. runnertg is of type std::string , while the element Runner::runnerTag is of type char[32] .

2. The expression runners[i] = new Runner({runnertg, timeStamp}); has another type mismatch. The element type of runners is Runner , while the expression new Runner({runnertg, timeStamp}) is of type Runner* .

3. runners[i] is out-of-bound access. The size of runners is 0. The elements runners[i] for all values of i does not exist.

4. There is memory leak since there is no matching delete for each new for all code path.

5. Don't use rand() .

Answer 2

A lot of the code you're using is old style C with some C++ STL code mixed in. I will try and tackle a few issues one at at time.

struct is an abstract data type used (in general) to organise primitive data types. While the only difference between a struct and a class is that the latter defaults all members to private and in this case is not functional, it's a good to cut these things of at the pass.

Secondly, an array of char s is cumbersome, messy and prone to error. Try an std::string instead.

Finally, let's create a constructor taking our two parameters.

Thus:

#include <string>

class Runner {
public:
    std::string runnerTag;
    uint32_t ts;

    Runner(std::string, uint32_t);
};

Next thing. Using an array index operator [] to access or modify an std::vector is dangerous and defeats the purpose of using a container and all the wonderful functionality that comes along with it.

Since you know for loops, let's try this:

for (int i = 0; i < 100; i++) {
    std::string runnertg = "testTrackTag01";
    uint32_t timeStamp = rand() % 100 + 1;
    Runner Runner(runnertg, timeStamp);
    runners.push_back(Runner);
}

At the end of your code, outside the scope of the main function, define the constructor as follows:

Runner::Runner(std::string rt,  uint32_t ts) {
    runnerTag = rt;
    ts = ts;
}

This should get you started.

Answer 3

You cannot convert std::string to a char array like that. Change the type of the member variable runnerTag to std::string . Also, you are accessing vector elements that don't exist. Use resize to create them. Or better yet, use emplace_back to do both at once. Also, don't use int to iterate containers, but std::size_t . Also, don't use rand() , but the classes from the <random> header. That trick using % creates a non-uniform distribution.