= default

Problem: Default Constructor Not Automatically Generated

Rule: C++ does not generate a default constructor if a constructor has been defined manually

code/c++11-default-missing.cpp

#include <iostream>
using namespace std;

class LackingDefaultConstructor
{
public:
    LackingDefaultConstructor(const std::string& s) : _s{s} {}
    const std::string& str() const { return _s; }
private:
    const std::string _s;
};

int main()
{
    LackingDefaultConstructor ldc;
    cout << ldc.str() << endl;;
    return 0;
}

code/c++11-default-explicit.cpp:15:31: error: no matching function for call to ‘LackingDefaultConstructor::LackingDefaultConstructor()’
   15 |     LackingDefaultConstructor ldc;
      |                               ^~~
code/c++11-default-explicit.cpp:7:5: note: candidate: ‘LackingDefaultConstructor::LackingDefaultConstructor(const string&)’
    7 |     LackingDefaultConstructor(const std::string& s) : _s{s} {}
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~
code/c++11-default-explicit.cpp:7:5: note:   candidate expects 1 argument, 0 provided
code/c++11-default-explicit.cpp:4:7: note: candidate: ‘LackingDefaultConstructor::LackingDefaultConstructor(const LackingDefaultConstructor&)’
    4 | class LackingDefaultConstructor
      |       ^~~~~~~~~~~~~~~~~~~~~~~~~

C++ < 11 Solution: Write Default Constructor 😠 👎

• Write possibly large amounts of code that could have generated by the compiler

• Well, a real-life default-ctor implementation might be a bit more complicated than in this example 🙄

code/c++11-default-explicit.cpp

#include <iostream>
using namespace std;

class HasDefaultConstructor
{
public:
    HasDefaultConstructor() {} // <---- HMPF!
    HasDefaultConstructor(const std::string& s) : _s{s} {}
    const std::string& str() const { return _s; }
private:
    const std::string _s;
};

int main()
{
    HasDefaultConstructor hdc;
    cout << hdc.str() << endl;;
    return 0;
}

C++ >= 11 Solution: = default

code/c++11-default-default.cpp

#include <iostream>
using namespace std;

class HasDefaultConstructor
{
public:
    HasDefaultConstructor() = default;
    HasDefaultConstructor(const std::string& s) : _s{s} {}
    const std::string& str() const { return _s; }
private:
    const std::string _s;
};

int main()
{
    HasDefaultConstructor hdc;
    cout << hdc.str() << endl;;
    return 0;
}