C++ vector passed by reference not modified

Question

When I pass a std::vector<std::string> by reference to a function which writes to the vector, the called function modifies the vector but the calling function doesn't see the modification.

For example, in the following code, oData has a size of 20 but data has size 0. Why?

#include <iostream>
#include <fstream>
#include <string>
#include <vector>

bool ReadFile(const std::string & iFile, std::vector<std::string> & oData) {

    std::ifstream myfile(iFile);

    if (!myfile.is_open()) {
        std::cout << "Unable to open file";
        return false;
    }
    else {
        std::string line;
        while (getline(myfile, line)) {
            std::cout << line << '\n';
            oData.push_back(line);
        }
        myfile.close();
    }
    return true;
}

int main() {
    std::vector<std::string> data;
    ReadFile("numbers.txt", data);

    return 0;
}

Note: Using Visual Studio 2015 on Windows 10

Answer 1

The problem was with the location of the breakpoint in Visual Studio debugger.

Breakpoint after return --> size of data = 0; screenshot

Breakpoint before return --> size of data = actual size. screenshot