[GiNaC-list] Converting an expression to std::string and back again

[Robert Hai]

Dear Mailing List,
 
for a project I need to convert a std::string to an ex, diff it and then convert it back to a std::string. For the conversion from std::string to ex I found this constructor : 
 
    /** Construct example from string and a list of symbols. The input grammar is * similar to the GiNaC 
    output format. All symbols and indices to be used * in the expression must be specified in a lst in the 
    second argument. * Undefined symbols and other parser errors will throw an exception. */ 
    ex(const std::string &s, const ex &l);

at http://www.ginac.de/reference/classGiNaC_1_1ex.html#ace68037326c1a2b8c93a56716599ec7a
 
So I tried doing the following :
 
    symbol x("x");
    ex expression;
    ex e("x^2",expression);
    std::cout << diff(e,x) << std::endl;

But this leads me to this exception :

std::invalid_argument: find_or_insert_symbol: symbol "x" not found

Note that I tried using expression and e in diff(e,x). Unfortunately I could not find any documentation on how to convert the expression diff returns to a std::string.

Thanks a lot for your help in advance ! 

Robert Hey