Questions related to use of C++ compiler-generated functions, generated implicitly, or explicitly by use of = default.
Questions tagged [defaulted-functions]
19 questions
22
votes
8 answers
What's the point in defaulting functions in C++11?
C++11 adds the ability for telling the compiler to create a default implementation of any of the special member functions. While I can see the value of deleting a function, where's the value of explicitly defaulting a function? Just leave it blank…
Motti
- 110,860
- 49
- 189
- 262
16
votes
1 answer
=default ignores access specifier?
I found it quite odd that the following program still compiled fine despite the default constructor being private (4.8.1 g++):
class A{
private:
A() = default;
A(const A&) = default;
};
int main(){
A a;
}
Actually from…
Silversonic
- 1,289
- 2
- 11
- 26
11
votes
1 answer
Can I default a private constructor in the class body or not?
GCC 4.5 doesn't let me do this:
class foo {
public:
foo() = default;
private:
foo(foo const&) = default;
foo& operator=(foo const&) = default;
};
It complains that:
error: 'foo::foo(const foo&)' declared with non-public access cannot…
R. Martinho Fernandes
- 228,013
- 71
- 433
- 510
8
votes
1 answer
Can special member functions be defaulted if they use typedefs?
Clang compiles this fine, but GCC and MSVC complain that operator= cannot be defaulted:
#include
template
struct S
{
typedef typename std::enable_if::value, S>::type Me;
S &operator=(Me const &) =…
user541686
- 205,094
- 128
- 528
- 886
8
votes
2 answers
Are user-defined default constructors less efficient?
Some days ago, while reading Standard C++ news I've read the post about Defaulted functions in C++11, in that article is mentioned that the user-defined constructor is less efficient than the one generated by the compiler:
The user-defined default…
PaperBirdMaster
- 12,806
- 9
- 48
- 94
7
votes
2 answers
Why is the constructor of std::in_place_t defaulted and explicit?
cppreference shows the following definition of std::in_place_t:
struct in_place_t {
explicit in_place_t() = default;
};
inline constexpr std::in_place_t in_place{};
Why have they added an explicit and defaulted constructor? Why it isn't left…
Martin Fehrs
- 792
- 4
- 13
7
votes
2 answers
Why would you "default" a copy/move constructor or a destructor?
C++0x lets you specify certain functions as defaulted:
struct A {
A() = default; // default ctor
A(A const&) = default; // copy ctor
A(A&&) = default; // move ctor
A(Other); // other ctor
~A() = default; …
Roger Pate
4
votes
3 answers
How can one default a special member function if one doesn't know its parameter types?
Consider this case:
template
struct A {
A(A ???&) = default;
A(A&&) { /* ... */ }
T t;
};
I explicitly declared a move constructor, so I need to explicitly declare a copy constructor if I want to have a non-deleted copy…
Johannes Schaub - litb
- 496,577
- 130
- 894
- 1,212
4
votes
0 answers
using std::unique_ptr pimpl with explicit default destructor
When defining the following class
class Foo
{
public:
Foo (void);
~Foo (void) = default;
protected:
class FooImpl;
std::unique_ptr _impl;
//...
};
Foo::Foo (void) : _impl (std::make_unique ()) {
}
I get the…
Daniel Heilper
- 1,182
- 2
- 17
- 34
3
votes
1 answer
Why C++20 allows defaulted comparison to compile even when it is deleted?
Consider the following code:
struct A {
};
struct B {
A a;
bool operator == (const B& other) const = default;
};
clang gives a nice warning :
warning: explicitly defaulted equality comparison operator is
implicitly deleted…
NoSenseEtAl
- 28,205
- 28
- 128
- 277
3
votes
3 answers
C++11 deleted/defaulted constructors
I'm a bit confused about how/why the constructors are called in C++11 and C++17.
#include
using namespace std;
//---
template
struct StructTest
{
public:
const T Var = -1;
//---
// constexpr StructTest() = delete; …
Andy
- 127
- 1
- 1
- 8
2
votes
1 answer
Why does a defaulted default constructor depend on whether it is declared inside or outside of class?
In the following example, struct A does not have default constructor. So both struct B and struct C inherited from it cannot get compiler-generated default constructor:
struct A {
A(int) {}
};
struct B : A {
B() = default; //#1
};
struct C…
Fedor
- 17,146
- 13
- 40
- 131
2
votes
1 answer
Visual Studio C2580 when using default keyword
For the following code:
struct S
{
S() = default;
S(S const&) = default;
S(S&&) = default;
S& operator=(S const& other) = default;
S& operator=(S&&) = default;
template
S(T&&... params)
{
…
Peter Lenkefi
- 1,306
- 11
- 29
1
vote
2 answers
Why does copy-and-swap in a base class cause the copy-assignment operator to be implicitly deleted in the derived class?
Tested only in GCC and Clang, the presence of a pass-by-value copy assignment operator in the base class (useful when implementing the copy-and-swap (or copy-and-move) idiom) causes the copy assignment operator in the derived class to be implicitly…
SumDood
- 294
- 1
- 7
1
vote
1 answer
C++ implicit definition of special functions
In the current version of the C++ draft (september 2019), paragraph [class.default.ctor]/4 states:
A default constructor that is defaulted and not defined as deleted is implicitly defined when it is odr-used ([basic.def.odr]) to create an object of…
user42768
- 1,951
- 11
- 22