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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Typische Funktion Erklärung

Umfassende Funktion Erklärung

Umfassende Funktion Erklärung mit nachgestellten Rückgabetyp

Typische nicht-Member-Funktion Definition

Umfassende Definition der Funktion

Umfassende Definition der Funktion mit nachgestellten Rückgabetyp

Rein virtuelle Memberfunktion

Defaulted Member-Funktion

Deleted Member-Funktion

#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