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error: template argument/substitution failed: mismatched types 'T (*)(V*)' and 'long double'

rcx11 (23)
I'm early in my work with templates. I've gotten a few to work, so I thought I understood them. However, this one is giving me some trouble.

Trying to call the following function (namespaces removed for length)

renew_hbar(nonlinear_sys_pressure(args), nonlinear_sys_gibbs(args), args, hbar);

where:
nonlinear_sys_pressure(args) & nonlinear_sys_gibbs(args) are functions returning long doubles and accepting an array of long doubles
args[3] & hbar[2] are long double arrays

The associated template is:
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template <typename T, typename U, typename V, typename W>
void renew_hbar(T (*f)(U[]), T (*g)(U[]), V args[], W hbar[])
{
    hbar[0] = hbar[0] - (a_i(f(args), g(args), args) + b_i(f(args), g(args), args));
    hbar[1] = hbar[1] - (c_i(f(args), g(args), args) + d_i(f(args), g(args), args));

    return;
}


I've previously succeeded in passing function pointers and arguments in a similar manner: 
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template <typename T, typename U, typename V>
T dn_dxn(int n, T (*f)(U[]), V args[], int narg)
{
    if (n == 0)
        return f(args);
    else
    {
        T h;
        if (sizeof(T) == 128)
            h = std::pow(args[narg] * std::sqrt(LDBL_EPSILON), 1 / n);
        else if (sizeof(T) == 64)
            h = std::pow(args[narg] * std::sqrt(DBL_EPSILON), 1 / n);
        else
            h = std::pow(args[narg] * std::sqrt(FLT_EPSILON), 1 / n);

        T sum = 0;

        for (int k = 0; k <= n; k++)
        {
            args[narg] += (n / 2 - k) * h;
            sum += std::pow(-1, k) * nCr(n, k) * f(args);
            args[narg] -= (n / 2 - k) * h;
        }

        return sum / std::pow(h, n);
    }
}


However, in my new case, I'm receiving an error that "template argument/substitution failed: mismatched types 'T (*)(V*)' and 'long double'"
I can tell this is an issue with the function pointer, but I've had no problems with it in my previous work. What's going on?
Cubbi (4774)
the type of the expression nonlinear_sys_pressure(args) is long double, so it can't be used as the first argument to renew_hbar, which expects a T (*f)(U[]).
You'd have to use nonlinear_sys_pressure (or &nonlinear_sys_pressure if you want to be explicit about pointer formation)

Last edited on
rcx11 (23)
That was the problem. I tried including the arguments in the function pass. I had it right before and just could see it. I've been staring at this code too long. Thank you!
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