Why do I get this error?

[Sophie]

class CVRUtilitiesApp : public CWinApp
{
public:


void CopySessions( const CString & src, const CString &dst,const CString & srcLesson, const CString & dstLesson, const vector<Session> &sessionToCopy);
void CopyLessons( const CString & src, const CString &dst, const vector<Lesson> &lessonToCopy);
void ToCopySession( const CString & src, const CString &dst, const CString & srcLesson, const CString & dstLesson, const vector<Session> &sessionToCopy);
void ToCopyLesson( const CString & src, const CString &dst, const vector<Lesson> &lessonToCopy);

bool GetLessonByName(CString &lessonName,const Lesson &lesson);
CVRUtilitiesApp();



void CVRUtilitiesApp::CopySessions( const CString & src, const CString &dst, const CString & srcLesson, const CString & dstLesson, const vector<Session> &sessionToCopy)
{
set<CString> tempLesson;
for( int j= 0; j < sessionToCopy.size(); j++) {
Directory destRepDir= sessionToCopy[j].GetPath().CalcMvPath((ToString(src)),(ToString(dst)));
CString lessonName = sessionToCopy[j].GetLessonName();
if(tempLesson.find(lessonName) == tempLesson.end()){
tempLesson.insert(lessonName);
Lesson l;
if(GetLessonByName(lessonName, l))
CopyLessons(srcLesson, dstLesson, l);

}

}
}
bool CVRUtilitiesApp::GetLessonByName(CString &lessonName, const Lesson &l)
{

for (int i=0; i< lessonRawList.size(); i++) {
if(lessonRawList[i].GetLessonName() = lessonName)
l = lessonRawList[i];
return true;
}

}

:\projet2\VRUtilities\VRUtilities.cpp(742) : error C2664: 'CopyLessons' : cannot convert parameter 3 from 'class Lesson' to 'const class std::vector<class Lesson,class std::allocator<class Lesson> > &'
Reason: cannot convert from 'class Lesson' to 'const class std::vector<class Lesson,class std::allocator<class Lesson> >'
No constructor could take the source type, or constructor overload resolution was ambiguous
K:\projet2\VRUtilities\VRUtilities.cpp(758) : error C2678: binary '=' : no operator defined which takes a left-hand operand of type 'const class Lesson' (or there is no acceptable conversion)

The first error message is caused by the call to CopyLessons. It expects the 3rd argument to be a vector of Lessons, and you are passing only a single Lesson. To call it as it stands you need to create a temporary vector<Lesson>, add your Lesson to it, and then pass the vector.

The other message is caused by the opposite problem. I can't see the declaration of lessonRawList, but since you have a loop from i=0 to lessonRawList.size(), I assume lessonRawList is a vector of Lessons. However GetLessonByName returns only a single Lesson. Within your loop over i, use lessonRawList [ i ] to refer to the current lesson.