Boost.Serialization: Error on Calling Class Method during serialization process - c++

Question

I have the following problem when I try to call method in "TestSerialize" class during serialization process.
Here is my code:
class TestSerialize
{
public:
std::string GetVal() { return Val + "abc"; }
void SetVal(std::string tVal) { Val = tVal.substr(0, 2); }
protected:
std::string Val;
friend class boost::serialization::access;
template<class Archive> void save(Archive & ar, const unsigned int version) const
{
using boost::serialization::make_nvp;
std::string tVal = GetVal(); // Error here
ar & make_nvp("SC", tVal);
}
template<class Archive> void load(Archive & ar, const unsigned int version)
{
using boost::serialization::make_nvp;
std::string tVal;
ar & make_nvp("SC", tVal);
SetVal(tVal);
}
BOOST_SERIALIZATION_SPLIT_MEMBER();
};
int main()
{
TestSerialize tS;
std::ofstream ofs("test.xml");
boost::archive::xml_oarchive oa(ofs, boost::archive::no_header);
oa << BOOST_SERIALIZATION_NVP(tS);
ofs.close();
return 0;
}
The error that I encountered is:
'TestSerialize::GetVal' : cannot convert 'this' pointer from 'const TestSerialize' to 'TestSerialize &'
This error only happens on "save" but not "load"
I wonder why I get this error. I would like to know what Boost.Serialization do such that we have these two different behaviors.
I use Boost Library 1.47.0

Answer 1

save is a const function and can only call other const functions. GetVal isn't. Change it:
std::string GetVal() const { ... }