C++: Vector of struct in header file -

i've got little question drives me crazy piece of cake guys. defined structure consists of 4 numbers , have function returns vector of structure. header file looks this:

legendre.hpp

#include <opencv2/opencv.hpp> #include <stdio.h> #include <stdlib.h> #include <iostream> #include <fstream> #include <sstream> #include <vector>  //as suggested, doesn't  #ifndef legendre_hpp_ #define legendre_hpp_      struct fourdoubles {      double zeroth;     double first;     double second;     double third; };  std::vector<fourdoubles> legendrelookuptable(int size);  #endif /* legendre_hpp_ */ 

(actually there more functions, ignore seemingly pointless includes) have following cpp file uses header:

legendre.cpp

#include "legendre.hpp"  using namespace std; using namespace cv;  vector<fourdoubles> legendrelookuptable(int size) {      vector<fourdoubles> res(size);     int i;     double x,x2,x3,x4;      for(i=0;i<size;i++) {      x=2.0*i/(size-1.0)-1.0;     x2=x*x;     x3=x2*x;     x4=x3*x;     res[i].zeroth=x;     res[i].first=1.5*x2-0.5;     res[i].second=2.5*x3-1.5*x;     res[i].third=4.375*x4-3.75*x2+0.375;     }  return res; } 

just in case curious: i'm calculating legendre moments of image. however, when want compile this, compiler tells me "field 'zeroth' not resolved", same other fields.

the error occurs when comment out else in files , when main looks

#include "legendre.hpp"  using namespace std;  int main(int argc, char* argv[]) { } 

the error simple d

escription  resource    path    location    type field 'third' not resolved legendre.cpp    /finder line 108    semantic error field 'second' not resolved    legendre.cpp    /finder line 107    semantic error field 'zeroth' not resolved    legendre.cpp    /finder line 105    semantic error field 'first' not resolved legendre.cpp    /finder line 106    semantic error 

(don't mind lines, said, commented out lot of stuff)

two things quite strange me: if don't declare vector, fourdoubles directly like

vector<fourdoubles> legendrelookuptable(int size) {     ...    fourdoubles test;    test.zeroth=5.5;    ... } 

then works fine. also, when write whole stuff in main file directly, e.g.

struct fourdoubles {      double zeroth;     double first;     double second;     double third; };  std::vector<fourdoubles> legendrelookuptable(int size);  int main() {     ... }  vector<fourdoubles> legendrelookuptable(int size) {    ... } 

i'm missing basic here, don't understand why can't handle vector of struct unless it's declared in main file. enlighten appreciated.

you've forgotten include <vector> header in hpp file. that's real source of error , that's why works if declare bare structure instead of vector of structures.