You need to decide on which Real type you want, perhaps #include <boost/multiprecision/cpp_bin_float.hpp> and then chose perhaps a quad precision type like cpp_bin_float_quad for 128-bit precision You can then either use a typedef to define Real typedef cpp_bin_float_quad Real; or pass cpp_bin_float_quad as a template parameter into your own function. HTH Paul PS It would be nice if we had a fully worked example? From: Boost-users <boost-users-bounces@lists.boost.org> On Behalf Of Anirban Pal via Boost-users Sent: 14 August 2020 01:08 To: boost-users@lists.boost.org Cc: Anirban Pal <anirban.pal.iitkgp@gmail.com> Subject: [Boost-users] Difficulty using boost quadrature library Hello everyone, I'm new to using boost and I'm trying to use its quadrature routines, specifically the tanh_sinh integrator. I'm trying to run the examples here: https://www.boost.org/doc/libs/1_66_0/libs/math/doc/html/math_toolkit/double... I've been successful using it with the double datatype. However, I can't seem to work with the Real datatype. When I run the following, I'm successful. tanh_sinh<double> integrator; auto f = [](double x) { return 5*x + 7; }; double Q = integrator.integrate(f); Q = integrator.integrate(f, 0.0, 1.1); But when I run the following: auto f = [](Real x) { return log(x)*log(1-x); }; Real Q = integrator.integrate(f, (Real) 0, (Real) 1); I get the error: boost_test.cxx: In function ‘int main(int, char**)’: boost_test.cxx:47:15: error: ‘Real’ has not been declared auto f2 = [](Real x) { return log(x)*log(1-x); }; ^~~~ boost_test.cxx:48:2: error: ‘Real’ was not declared in this scope Real Q2 = integrator.integrate(f2, (Real) 0, (Real) 1); I think I need to include the right headers for this to go away. However, I'm not sure what the correct header file is. Thank you for your attention. -- Anirban Pal