# External C++ (experimental)¶

This feature is experimental and nothing is guaranteed.

Instructions follow loosely https://dfm.io/posts/stan-c++/ which gives more in-depth example using external C++.

## Setting up C++¶

Your C++ code needs to provide needed gradients and partial derivatives to work properly. This can be done by manually implementing each needed component or by using Stan’s autodiff (special syntax). Each function also needs to accept a std::ostream as a last argument.

In the following instructions it is assumed that the first code is saved to a file called external_manual.hpp and external_autograd.hpp. Both files are saved under cwd in a directory external_cpp.

The following code shows a minimal example with manually implemented gradients for a function C = A*B*B+A:

inline double my_func (double A, double B, std::ostream* pstream) {
double C = A*B*B+A;

return C;
}

inline var my_func (const var& A_var, const var& B_var, std::ostream* pstream) {
// Compute the value
double A = A_var.val(),
B = B_var.val(),
C = my_func(A, B, pstream);

// Compute the partial derivatives:
double dC_dA = B*B+1.0,
dC_dB = 2.0*A*B;

// Autodiff wrapper:
return var(new precomp_vv_vari(
C,             // The _value_ of the output
A_var.vi_,     // The input gradient wrt A
B_var.vi_,     // The input gradient wrt B
dC_dA,         // The partial introduced by this function wrt A
dC_dB          // The partial introduced by this function wrt B
));
}

inline var my_func (double A, const var& B_var, std::ostream* pstream) {
double B = B_var.val(),
C = my_func(A, B, pstream),
dC_dB = 2.0*A*B;
return var(new precomp_v_vari(C, B_var.vi_, dC_dB));
}

inline var my_func (const var& A_var, double B, std::ostream* pstream) {
double A = A_var.val(),
C = my_func(A, B, pstream),
dC_dA = B*B+1.0;
return var(new precomp_v_vari(C, A_var.vi_, dC_dA));
}


The minimal example using Stan´s autograd for a function C = A*B*B+A:

template <typename T1, typename T2>
typename boost::math::tools::promote_args<T1, T2>::type
my_other_func (const T1& A, const T2& B, std::ostream* pstream) {
typedef typename boost::math::tools::promote_args<T1, T2>::type T;

T C = A*B*B+A;

return C;
}


## Setting up Stan¶

User needs to define functions inside a functions block. Function names are defined in the C++ code.

functions {
real my_func(real A, real B);
real my_other_func(real A, real B);
}
data {
real B;
}
parameters {
real A;
real D;
}
transformed parameters {
real C = my_func(A, B);
real E = my_other_func(D, B);
}
model {
C ~ std_normal();
E ~ std_normal();
}


## Setting up Python¶

In Python user needs information of location for external C++ code. This information is passed to StanModel with two list-objects include_dir and includes. Also allow_undefined keyword is set to True.

import pystan
import os

model_code = """...
"""

include_dir = [os.path.join(".", "external_cpp")]

Compilation with external C++ can take longer than normally. The external C++ code is injected to the stanc translated C++ code. This injections is done automatically by PyStan before model compilation.