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I have small problem, because I dont know how write my template method with compare cmp like optional parametr (compare cmp is function, not variable).

It is very difficult for me, because I am new about templates in c++ and new in c++ generaly :D

I want, that when I dont use compare src, compare src return true (beacuse of if condition...) Any ideas, please? :)

   template<typename compare>
   map<_NODE, int> Find(const _NODE& src, const int& max, const compare& cmp) const {
      map<_NODE, int> finded;
      queue<_NODE> q;
      _NODE town;
      //src doesn't exists
      if (mapTown.find(src) == mapTown.end()) {
         ostringstream oss;
         oss << "unknown " << src;
         throw invalid_argument(oss.str());
      //begin of queue
      finded.insert(make_pair(src, 0));
      //BFS algorithm
      while (!q.empty()) {
         town = q.front();
         //we are looking for all neighborhood of town in max length
         for (const pair<_NODE, list<_EDGE>> i : (mapTown.at(town))) {
            for (const _EDGE j : i.second) {
               if (finded.count(i.first) == 0 && finded[town] < max && cmp(j)) {
                  finded.insert(make_pair(i.first, finded[town] + 1));
      return finded;

C - Functions, For member functions of class templates, all defaults must be provided in the initial declaration of the member function. class C { void f(int i = 3)  A function definition in C programming consists of a function header and a function body. Here are all the parts of a function − Return Type − A function may return a value. The return_type is the data type of the value the function returns. Some functions perform the desired operations without returning a value.

If compare is a typedef of a function pointer you could pass nullptr as default option and inside your code simply check first if function exists, if not, default to true:

#include <iostream>

typedef bool(*DefaultComparator)(int);

bool NumberIsEven(int i)
    return i % 2 == 0;

template<typename compare = DefaultComparator>
void CheckNumber(int j, compare cmp = nullptr)
    if (j > 0 && (cmp ? cmp(j) : true))
        std::cout << "checked" << std::endl;
        std::cout << "failed" << std::endl;

int main()
    CheckNumber(1, NumberIsEven);
    CheckNumber(2, NumberIsEven);



Default arguments, In C, if you define a function in a header file, then that function will appear in each module that is compiled that includes that header file, and a public symbol will  2. Method Overloading To make a parameter optional, we can use method overloading. Using method overloading, we can create several methods with the same name but with a different method signature. So, first let's create a method in which the user can pass n numbers. The preceding code is the same as what we have seen in the params int[] demo.

First, as others mentioned, please post relelant code as Minimal, Complete, and Verifiable example. Also as others suggested templates are probably not a good idea to start learning c++. But if you are determined you should try on something more easier to get the grasp of concepts.

As it goes for your problem, you have at least two options for how to write a function which receives a comparator. More modern approach is by using c++11's functional library and its std::function type. Second approach is by using templates, in this case an function pointer is passed to the function.

To demonstrate both options here is a minimal example:

#include <functional>

bool evaluate(int value) {
  return value > 0;

void function1(int value, std::function<bool(int)> eval = evaluate) {
  bool result = eval(value);

template<class Type = decltype(evaluate)>
void function2(int value, Type eval = evaluate) {
  bool result = eval(value);

int main() {
  return 0;

In the example both options receive one integer and the evaluator returns true if the integer is greater than 0. Also in both functions the evaluator is defaulted to predefined evaluate function and it can be checked (if it exists) by:

if(eval) {

Why can you have the method definition inside the header file in , A default argument is a default value provided for a function parameter. declaration is more likely to be seen by other files (particularly if it's in a header file). In C++, functions can also have optional parameters, for which no arguments are required in the call, in such a way that, for example, a function with three parameters may be called with only two. For this, the function shall include a default value for its last parameter, which is used by the function when called with fewer arguments.

The existing answers will make code that works, but as a rule, with templated functions, you don't want them to be templated on function pointers, because that inhibits optimizations. In this case, inhibiting optimizations is a (relatively) big deal, because your desired default case makes your "compare" function (really a predicate function, since it's testing a condition on a single argument, not comparing multiple arguments) a no-op; if properly optimized, the default use of your function should not call your "always true" predicate at all, because it would inline the constant result, and simplify your test condition.

To properly generate optimized templates, you want to pass functors or C++11 lambdas (which are largely syntactic sugar for functors) as your predicate. This also means making the default predicate a lambda or functor, so the default case is fully performant. Thanks to C++11 lambdas, this is pretty easy:

// Define a trivial predicate that return true for any int input
// lambdas can only be constexpr on C++17 and higher; for C++11/C++14, you can
// just declare it const, and optionally static in either case
constexpr auto alwaystrue = [](int) { return true; };

// The type of a lambda is accessible via decltype, make that the default type
// of Predicate, so you can make the default value of predicate alwaystrue itself
template<typename Predicate=decltype(alwaystrue)>
map<_NODE, int> Find(const _NODE& src, const int max, Predicate pred=alwaystrue) const {

This is a significantly better option than accepting a function pointer defaulting to nullptr or an existing alwaystrue function; accepting a function pointer defaulting to nullptr means you need to test it for nullptr over and over (or split the code path into "given predicate" vs. "not given predicate"; if you're lucky, the compiler might do that for you, but don't count on it), and accepting as a function pointer at all means the function call can't be omitted, and must be performed through a pointer, not a stable jump to a known function location, meaning all the function call setup/call/return/teardown overhead is paid every time, even if the work to be done is a constant return true;.

By writing it this way, even minimal optimizations (-O1) ensure that the default predicate of alwaystrue is optimized away entirely (it behaves as if that specialization of Find doesn't even include && cmp(j)/&& pred(j)). If the caller explicitly passes a function/function pointer, they'll usually (barring extreme link time optimizations) get the slower behavior associated with calls through function pointer but it will still work; if they know what they're doing, they'll pass a lambda or functor and get the same optimizations alwaystrue gets (inlining the test to avoid function call overhead completely, though obviously it can't optimize the actual test itself unless its as a simple as alwaystrue's test).

As an alternative approach that avoids spilling information about alwaystrue to anyone who includes your header, you can define two versions of your function, one of which accepts the predicate, and the other of which does not. Done this way, you can hide alwaystrue in your source file (or inline the raw lambda without naming it), where it won't be seen or shared with other compilation units. Done this way, you can define a non-defaulting templated function and declare (without defining) a non-templated function that doesn't take a predicate argument in your header:

template<typename Predicate>
map<_NODE, int> Find(const _NODE& src, const int max, Predicate pred) const {

map<_NODE, int> Find(const _NODE& src, const int max) const;

then in your source file (hidden from others including your header), you'd the trivial body of the non-templated function implemented in terms of the templated function:

map<_NODE, int> Find(const _NODE& src, const int max) const {
    return Find(src, max, [](int) { return true; });

This approach is what the C++ standard library does with functions like std::sort where the comparator is optional; template< class RandomIt > void sort( RandomIt first, RandomIt last ); is typically implemented as return sort(first, last, functor_to_compare_iterator_targets());.

7.7, Below is a simple C/C++ program to demonstrate functions. Putting parameter names in function declaration is optional in the function declaration, but it is The library functions are declared in header files and defined in library files. Pass by Value: In this parameter passing method, values of actual parameters are  A common use case for optional is the return value of a function that may fail. As opposed to other approaches, such as std::pair<T,bool>, optional handles expensive-to-construct objects well and is more readable, as the intent is expressed explicitly.

Functions in C/C++, In C++, a function is a group of statements that is given a name, and which can be called from This value is defined in header <cstdlib> . In C++, functions can also have optional parameters, for which no arguments are required in the call,  Optional arguments enable you to omit arguments for some parameters. Both techniques can be used with methods, indexers, constructors, and delegates. When you use named and optional arguments, the arguments are evaluated in the order in which they appear in the argument list, not the parameter list.

Functions - C++ Tutorials, Set default values for function arguments. using the c function, e.g. x <- c("A", "B​", "C") creates a vector x with three elements. dat <- read.csv(file = "data/​inflammation-01.csv", header = FALSE) centered <- center(dat[, 4], 0) head(​centered) You will want to switch to this more formal method of writing documentation  Class Methods. Methods are functions that belongs to the class. There are two ways to define functions that belongs to a class: Inside class definition; Outside class definition; In the following example, we define a function inside the class, and we name it "myMethod".

Creating Functions – Programming with R, In JavaScript, every parameter is optional, and users may leave them off as they see fit. A top-level non-method syntax call like this will use window for this . Functions in the C programming Language . The C language is similar to most modern programming languages in that it allows the use of functions, self contained "modules" of code that take inputs, do a computation, and produce outputs. C functions must be TYPED (the return type and the type of all parameters specified).

  • @melpomene There is no even picture of code
  • @freedom266 What is your question? Please show us the relevant code - minimal reproducible example. And as a side note, templates might not be the best place to start learning C++
  • @melpomene Oops, my bad. Sorry.
  • @melpomene My bad, I posted the code now :D
  • What types can compare be?
  • "You are not supposed to name your own constructs with names beginning with an underscore, e.g. _NODE; that's reserved for the compiler and the standard library." Not exactly. Compiler reserved names start with double underscore or with underscore and upper letter (which is the case actually). Otherwise it's just ugly and not recommended way to denote private members.
  • This is school homework and name Find is stock from professor and he send some asserts to this function, so rules are very strict abou implementation :)