How to print / check value of vector in C++

Question

I just want to check the value of the variable as the code runs (like you can do in Matlab by just typing the name of the variable). I inserted one line of code cout << img_names; in a sample open source code but got no operator "<<" matches these operands in error list of visual studio. However, I see cout being used fine in other parts of the sample code. Below is the beginning part of the sample code where cout is inserted:

#include <iostream>
#include <fstream>
#include <string>
#include "opencv2/opencv_modules.hpp"
#include <opencv2/core/utility.hpp>
#include "opencv2/imgcodecs.hpp"
#include "opencv2/highgui.hpp"
#include "opencv2/stitching/detail/autocalib.hpp"
#include "opencv2/stitching/detail/blenders.hpp"
#include "opencv2/stitching/detail/timelapsers.hpp"
#include "opencv2/stitching/detail/camera.hpp"
#include "opencv2/stitching/detail/exposure_compensate.hpp"
#include "opencv2/stitching/detail/matchers.hpp"
#include "opencv2/stitching/detail/motion_estimators.hpp"
#include "opencv2/stitching/detail/seam_finders.hpp"
#include "opencv2/stitching/detail/util.hpp"
#include "opencv2/stitching/detail/warpers.hpp"
#include "opencv2/stitching/warpers.hpp"

using namespace std;
using namespace cv;
using namespace cv::detail;

static void printUsage(){ ... }

// Default command line args
vector<String> img_names;
bool preview = false;
bool try_cuda = false;
double work_megapix = 0.6;
double seam_megapix = 0.1;
double compose_megapix = -1;
float conf_thresh = 1.f;
string features_type = "surf";
string ba_cost_func = "ray";
string ba_refine_mask = "xxxxx";
bool do_wave_correct = true;
WaveCorrectKind wave_correct = detail::WAVE_CORRECT_HORIZ;
bool save_graph = false;
std::string save_graph_to;
string warp_type = "spherical";
int expos_comp_type = ExposureCompensator::GAIN_BLOCKS;
float match_conf = 0.3f;
string seam_find_type = "gc_color";
int blend_type = Blender::MULTI_BAND;
int timelapse_type = Timelapser::AS_IS;
float blend_strength = 5;
string result_name = "result.jpg";
bool timelapse = false;
int range_width = -1;

static int parseCmdArgs(int argc, char** argv){ ... }

int main(int argc, char* argv[])
{
#if ENABLE_LOG
    int64 app_start_time = getTickCount();
#endif

#if 0
    cv::setBreakOnError(true);
#endif

    int retval = parseCmdArgs(argc, argv);
    cout << img_names;
    if (retval)
        return retval;

much more code continues...

How do I use cout properly to get the variable values or is there an easier way?

Answer 1

operator << is not defined for std::vector because there is no standard way to print that type. You can implement your own like so

std::ostream& operator<< (std::ostream& out, const std::vector<String>& vec)
{
    for (int i = 0; i < vec.size(); i++)
        out << vec[i] << " "; // or whatever formatting you like

    return out;
}