Builder
12345678910111213141516171819202122232425262728293031323334353637383940414243 | #include <vector>
class foo
{
public:
class builder;
foo(int prop1, bool prop2, bool prop3, std::vector<int> prop4)
: prop1{prop1}, prop2{prop2}, prop3{prop3}, prop4{prop4}
{ }
int prop1;
bool prop2;
bool prop3;
std::vector<int> prop4;
};
class foo::builder
{
public:
builder& set_prop1(int value) { prop1 = value; return *this; };
builder& set_prop2(bool value) { prop2 = value; return *this; };
builder& set_prop3(bool value) { prop3 = value; return *this; };
builder& set_prop4(std::vector<int> value) { prop4 = value; return *this; };
foo build() const
{
return foo{prop1, prop2, prop3, prop4};
}
private:
int prop1 = 0;
bool prop2 = false;
bool prop3 = false;
std::vector<int> prop4 = {};
};
int main()
{
foo f = foo::builder{}.set_prop1(5)
.set_prop3(true)
.build();
} |
This pattern is licensed under the CC0 Public Domain Dedication.
Intent
Separate the complex construction of an object from its representation.
Description
The foo class, on lines 3–16, has a complex construction process
during which any subset of its properties might be set. This
process is captured by the foo::builder class, on lines 18–36.
This builder class provides an interface for
constructing foo objects, allowing various combinations of
parameters to be provided. This avoids having to define a large
collection of constructors for foo.
The foo::builder class implements a set of
chainable functions for setting the construction parameters
(lines 21–24) and a build member function for constructing the foo
object with these parameters (lines 26–29).
On lines 40–42, we use foo::builder to construct a foo object,
setting its prop1 and prop3 members and calling build to
construct the object.