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#define _USE_MATH_DEFINES
 
#include <iostream>
#include <cmath>
#include <functional>
 
using namespace std;
 
#define MIN -10.
#define MAX 10.
#define EPS 0.01
 
template <typename T, class F> T trapezoid(F f, T a, T b, T eps) {
    T result = T(0);
    while (a <= b) {
        result += f(a);
        a += eps;
    }
    result -= (f(a) + f(b)) / T(2);
    return result * eps;
}
 
template <class F>
double splot(F g, F l, double x, double sigma, double gamma) {
//template <class F1, class F2>
//double splot(F1 g, F2 l, double x, double sigma, double gamma) {
 
    cout << typeid(g).name()<<" vs." <<typeid(g).name()<<endl; 
 
    auto s = [g, l, x, sigma, gamma](double x_prime)->double {
        return g(x_prime, sigma) * l(x - x_prime, gamma);
    };
 
    return trapezoid(s, MIN, MAX, EPS);
}
 
int main (void) {
    double x = 0., sigma = 1.5, gamma = 0.1;
 
    auto gauss = [](double x, double sigma)->double {
        return exp(-1*x*x / (2*sigma*sigma)) / (sigma * sqrt(2*M_PI));
    };
 
    auto lorentz = [](double x, double gamma)->double {
        return gamma / (M_PI*(x*x + gamma*gamma));
    };
 
    cout << "x: " << x << endl << "V(x): " <<
     splot(gauss, lorentz, x, sigma, gamma) << endl;
 
    return 0;
}

prog.cpp: In function ‘int main()’:
prog.cpp:49:43: error: no matching function for call to ‘splot(main()::<lambda(double, double)>&, main()::<lambda(double, double)>&, double&, double&, double&)’
      splot(gauss, lorentz, x, sigma, gamma) << endl;
                                           ^
prog.cpp:24:8: note: candidate: template<class F> double splot(F, F, double, double, double)
 double splot(F g, F l, double x, double sigma, double gamma) {
        ^~~~~
prog.cpp:24:8: note:   template argument deduction/substitution failed:
prog.cpp:49:43: note:   deduced conflicting types for parameter ‘F’ (‘main()::<lambda(double, double)>’ and ‘main()::<lambda(double, double)>’)
      splot(gauss, lorentz, x, sigma, gamma) << endl;
                                           ^

Standard output is empty