google::cloud::Options Class Reference

A class that holds option structs indexed by their type. More...

#include <google/cloud/options.h>

Public Member Functions
	Options ()=default
	Constructs an empty instance. More...
	Options (Options const &rhs)
Options &	operator= (Options const &rhs)
	Options (Options &&)=default
Options &	operator= (Options &&)=default
template<typename T >
Options &	set (ValueTypeT< T > v)
	Sets option T to the value v and returns a reference to *this. More...
template<typename T >
bool	has () const
	Returns true IFF an option with type T exists. More...
template<typename T >
void	unset ()
	Erases the option specified by the type T. More...
template<typename T >
ValueTypeT< T > const &	get () const
	Returns a reference to the value for T, or a value-initialized default if T was not set. More...
template<typename T >
ValueTypeT< T > &	lookup (ValueTypeT< T > value={})
	Returns a reference to the value for option T, setting the value to init_value if necessary. More...

Detailed Description

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"});

Constructor & Destructor Documentation

Options() [1/3]

google::cloud::Options::Options ( )

default

Constructs an empty instance.

Options() [2/3]

google::cloud::Options::Options ( Options const &  rhs )

inline

Options() [3/3]

google::cloud::Options::Options ( Options &&  )

default

Member Function Documentation

get()

template<typename T >

ValueTypeT< T > const & google::cloud::Options::get ( ) const

inline

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);

Template Parameters

T	the option type

has()

template<typename T >

bool google::cloud::Options::has ( ) const

inline

Returns true IFF an option with type T exists.

Template Parameters

T	the option type

lookup()

template<typename T >

ValueTypeT< T > & google::cloud::Options::lookup ( ValueTypeT< T >  value = {} )

inline

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);

Template Parameters

T	the option type

Parameters

value

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

operator=() [1/2]

Options & google::cloud::Options::operator= ( Options &&  )

default

operator=() [2/2]

Options & google::cloud::Options::operator= ( Options const &  rhs )

inline

set()

template<typename T >

Options & google::cloud::Options::set ( ValueTypeT< T >  v )

inline

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

struct FooOption {
  using Type = int;
};
auto opts = Options{}.set<FooOption>(123);

Template Parameters

T	the option type

Parameters

v	the value to set the option T

unset()

template<typename T >

void google::cloud::Options::unset ( )

inline

Erases the option specified by the type T.

Template Parameters

T	the option type