C++ Classes and Templates - 2021.2 English

Vitis High-Level Synthesis User Guide (UG1399)

Document ID
English (United States)
Release Date
2021.2 English

C++ classes are fully supported for synthesis with Vitis HLS. The top-level for synthesis must be a function. A class cannot be the top-level for synthesis. To synthesize a class member function, instantiate the class itself into function. Do not simply instantiate the top-level class into the test bench. The following code example shows how class CFir (defined in the header file discussed next) is instantiated in the top-level function cpp_FIR and used to implement an FIR filter.

#include "cpp_FIR.h"

// Top-level function with class instantiated
data_t cpp_FIR(data_t x)
 static CFir<coef_t, data_t, acc_t> fir1;

 cout << fir1;

 return fir1(x);
Important: Classes and class member functions cannot be the top-level for synthesis. Instantiate the class in a top-level function.

Before examining the class used to implement the design in the C++ FIR Filter example above, it is worth noting Vitis HLS ignores the standard output stream cout during synthesis. When synthesized, Vitis HLS issues the following warnings:

INFO [SYNCHK-101] Discarding unsynthesizable system call: 
'std::ostream::operator<<' (cpp_FIR.h:108)
INFO [SYNCHK-101] Discarding unsynthesizable system call: 
'std::ostream::operator<<' (cpp_FIR.h:108)
INFO [SYNCHK-101] Discarding unsynthesizable system call: 'std::operator<< 
<std::char_traits<char> >' (cpp_FIR.h:110)

The following code example shows the header file cpp_FIR.h, including the definition of class CFir and its associated member functions. In this example the operator member functions () and << are overloaded operators, which are respectively used to execute the main algorithm and used with cout to format the data for display during C/C++ simulation.

#include <fstream>
#include <iostream>
#include <iomanip>
#include <cstdlib>
using namespace std;

#define N 85

typedef int coef_t;
typedef int data_t;
typedef int acc_t;

// Class CFir definition
template<class coef_T, class data_T, class acc_T>
class CFir {
   static const coef_T c[N];
   data_T shift_reg[N-1];
   data_T operator()(data_T x);
   template<class coef_TT, class data_TT, class acc_TT>
   friend ostream&
   operator<<(ostream& o, const CFir<coef_TT, data_TT, acc_TT> &f);

// Load FIR coefficients
template<class coef_T, class data_T, class acc_T>
const coef_T CFir<coef_T, data_T, acc_T>::c[N] = {
 #include "cpp_FIR.h"

// FIR main algorithm
template<class coef_T, class data_T, class acc_T>
data_T CFir<coef_T, data_T, acc_T>::operator()(data_T x) {
 int i;
 acc_t acc = 0;
 data_t m;

 loop: for (i = N-1; i >= 0; i--) {
   if (i == 0) {
      m = x;
      shift_reg[0] = x;
   } else {
      m = shift_reg[i-1];
      if (i != (N-1))
      shift_reg[i] = shift_reg[i - 1];
   acc += m * c[i];
 return acc;

// Operator for displaying results
template<class coef_T, class data_T, class acc_T>
ostream& operator<<(ostream& o, const CFir<coef_T, data_T, acc_T> &f) {
 for (int i = 0; i < (sizeof(f.shift_reg)/sizeof(data_T)); i++) {
    o << shift_reg[ << i << ]=  << f.shift_reg[i] << endl;
 o << ______________ << endl;
 return o;

data_t cpp_FIR(data_t x);

The test bench in the C++ FIR Filter example is shown in the following code example and demonstrates how top-level function cpp_FIR is called and validated. This example highlights some of the important attributes of a good test bench for Vitis HLS synthesis:

  • The output results are checked against known good values.
  • The test bench returns 0 if the results are confirmed to be correct.
#include "cpp_FIR.h"

int main() {
 ofstream result;
 data_t output;
 int retval=0;

 // Open a file to saves the results

 // Apply stimuli, call the top-level function and saves the results
 for (int i = 0; i <= 250; i++)
    output = cpp_FIR(i);

    result << setw(10) << i;
    result << setw(20) << output;
    result << endl;


 // Compare the results file with the golden results
 retval = system(diff --brief -w result.dat result.golden.dat);
 if (retval != 0) {
    printf(Test failed  !!!\n); 
 } else {
    printf(Test passed !\n);

 // Return 0 if the test
 return retval;

C++ Test Bench for cpp_FIR

To apply directives to objects defined in a class:

  1. Open the file where the class is defined (typically a header file).
  2. Apply the directive using the Directives tab.

As with functions, all instances of a class have the same optimizations applied to them.