返回lambda表达式的方法中的转换错误

#include "iostream"
#include  "functional"
std::function<double (double)> retFun(double* a) {
     return [a](double x) { return x+*a; };
 }

 int main(){

 double*e = new double(3);
 std::cout << retFun(e)(3) << std::endl;}

retFun 在对象中:

.h

class InternalVariables : public Page
{
private:
    std::function<double ()> retFun(double* a);
};

.清洁石油产品

std::function<double ()> InternalVariables::retFun(double *a)
{
    return [a](double b){ return b+ *a;};
}

我得到以下错误

错误:无法将InternalVariables::retFun(double*)::_ulambda44{a}从InternalVariables::retFun(double*):::_ulambda44转换为std::函数 return[a](双b){return b+*a;};