Class Options (2.34.0-rc)

A class that holds option structs indexed by their type.

An "Option" is any struct that has a public Type member typedef. By convention they are named like "FooOption". Each library (e.g., spanner, storage) may define their own set of options. Additionally, there are common options defined that many libraries may use. All these options may be set in a single Options instance, and each library will look at the options that it needs.

Here's an overview of this class's interface, but see the method documentation below for details.

  • .set<T>(x)– Sets the option T to value x
  • .has<T>()– Returns true iff option T is set
  • .unset<T>()– Removes the option T
  • .get<T>()– Gets a const-ref to the value of option T
  • .lookup<T>(x)– Gets a non-const-ref to option T's value, initializing it to x if it was not set (x is optional).
Example:
struct FooOption {
  using Type = int;
};
struct BarOption {
  using Type = std::set<std::string>;
};
...
Options opts;

assert(opts.get<FooOption>() == 0);
opts.set<FooOption>(42);
assert(opts.get<FooOption>() == 42);

// Inserts two elements directly into the BarOption's std::set.
opts.lookup<BarOption>().insert("hello");
opts.lookup<BarOption>().insert("world");

std::set<std::string> const& bar = opts.get<BarOption>();
assert(bar == std::set<std::string>{"hello", "world"});

Constructors

Options()

Constructs an empty instance.

Options(Options const &)

Parameter
Name Description
rhs Options const &

Options(Options &&)

Parameter
Name Description
rhs Options &&

Operators

operator=(Options const &)

Parameter
Name Description
rhs Options const &
Returns
Type Description
Options &

operator=(Options &&)

Parameter
Name Description
rhs Options &&
Returns
Type Description
Options &

Functions

set(ValueTypeT< T >) &

Sets option T to the value v and returns a reference to *this. 

struct FooOption {
  using Type = int;
};
auto opts = Options{};
opts.set<FooOption>(123);
Parameters
Name Description
v ValueTypeT< T >

the value to set the option T

typename T

the option type

Returns
Type Description
Options &

set(ValueTypeT< T >) &&

Sets option T to the value v and returns a reference to *this. 

struct FooOption {
  using Type = int;
};
auto opts = Options{}.set<FooOption>(123);
Parameters
Name Description
v ValueTypeT< T >

the value to set the option T

typename T

the option type

Returns
Type Description
Options &&

has() const

Returns true IFF an option with type T exists.

Parameter
Name Description
typename T

the option type

Returns
Type Description
bool

unset()

Erases the option specified by the type T.

Parameter
Name Description
typename T

the option type

Returns
Type Description
void

get() const

Returns a reference to the value for T, or a value-initialized default if T was not set.

This method will always return a reference to a valid value of the correct type for option T, whether or not T has actually been set. Use has<T>() to check whether or not the option has been set.

struct FooOption {
  using Type = std::set<std::string>;
};
Options opts;
std::set<std::string> const& x = opts.get<FooOption>();
assert(x.empty());
assert(!x.has<FooOption>());

opts.set<FooOption>({"foo"});
assert(opts.get<FooOption>().size() == 1);
Parameter
Name Description
typename T

the option type

Returns
Type Description
ValueTypeT< T > const &

lookup(ValueTypeT< T >)

Returns a reference to the value for option T, setting the value to init_value if necessary. 

struct BigOption {
  using Type = std::set<std::string>;
};
Options opts;
std::set<std::string>& x = opts.lookup<BigOption>();
assert(x.empty());

x.insert("foo");
opts.lookup<BigOption>().insert("bar");
assert(x.size() == 2);
Parameters
Name Description
value ValueTypeT< T >

the initial value to use if T is not set (optional)

typename T

the option type

Returns
Type Description
ValueTypeT< T > &