Declaring functions - cppreference.com

Original:

A function declaration introduces the function name and its type. It may appear in any scope, and is commonly placed in header files.

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Original:

A function definition provides the body of a function. It may only appear in namespace or class scope.

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Dichiarazione di funzione tipica

Dichiarazione di funzione completa

Dichiarazione di funzione completa con tipo di ritorno finale

Tipico non membro funzione definizione

Definizione di funzione completa

Definizione di funzione completa con tipo di ritorno finale

Pure funzione membro virtuale

Funzione membro inadempiente

Soppresso funzione membro

#include <iostream>
#include <string>

// declaration in namespace(file) scope
// (the definition is provided later)
int f1();

// simple function with a default argument, returning nothing
void f0(const std::string& arg = "world") {
    std::cout << "Hello, " << arg << '\n';
}

// function returning a pointer to f0
auto fp11() -> void(*)(const std::string&) {
    return f0;
}

// function returning a pointer to f0, pre-C++11 style
void (*fp03())(const std::string&) {
    return f0;
}

int main()
{
    f0();
    fp11()("test");
    fp03()("again");
    int f2(std::string); // declaration in function scope
    std::cout << f2("bad12") << '\n';
}

// simple non-member function returning int
int f1() {
    return 42;
}

// function with an exception specification and a function try block
int f2(std::string str) noexcept try { 
    return std::stoi(str);
} catch(const std::exception& e) {
    std::cerr << "stoi() failed!\n";
    return 0;
}

Hello, world
Hello, test
Hello, again
stoi() failed!
0

#include <iostream>
#include <string>
#include <utility>
#include <exception>

struct S {
    int data;

    // simple converting constructor (declaration)
    S(int val);

    // simple explicit constructor (declaration)
    explicit S(std::string str);

    // const member function (definition)
    virtual int getData() const { return data; }
    
};

// definition of the constructor
S::S(int val) : data(val) {
    std::cout << "ctor1 called, data = " << data << '\n';
}

// this constructor has a catch clause
S::S(std::string str) try : data(std::stoi(str)) {
    std::cout << "ctor2 called, data = " << data << '\n';
} catch(const std::exception&) {
    std::cout << "ctor2 failed, string was '" << str << "'\n";
    throw; // ctor's catch clause should always rethrow
}

struct D : S {
    int data2;
    // constructor with a default argument
    D(int v1, int v2 = 11) : S(v1), data2(v2) {}

    // virtual member function
    int getData() const override { return data*data2; }

    // lvalue-only assignment operator
    D& operator=(D other) & {
        std::swap(other.data, data);
        std::swap(other.data2, data2);
        return *this;
    }
};

int main()
{
    D d1 = 1;
    S s2("2");
    try {
         S s3("not a number");
    } catch(const std::exception&) {}
    std::cout << s2.getData() << '\n';
    
   D d2(3, 4);
   d2 = d1; // OK: assignment to lvalue
//   D(5) = d1; // ERROR: no suitable overload of operator=
}

ctor1 called, data = 1
ctor2 called, data = 2
ctor2 failed, string was 'not a number'
2
ctor1 called, data = 3