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I'm writing some code using std::optional's and am wondering if C++17's 'if statements with initializers' will be able to help unpack values?
std::optional<int> optionalInt = GetOptionalInt();
I'm making up the function Unpack here:
if( auto [value, has_value] = optionalInt.Unpack(); has_value )
{
// Use value here.
}
But, my question is. Will C++17 'if statement with initializer' help here? If so, how would it be coded?
Update, this is actually mainly an issue when using optional which is extremely easy to misuse because the optional and *optional both return bools and you don't get any compiler warning when somebody trys to access the value and forgets the *.
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C++17 If statement with initializer Now it is possible to provide initial condition within if statement itself. This new syntax is called "if statement with initializer". This enhancement simplifies common code patterns and helps users keep scopes tight.
The class template std::optional manages an optional contained value, i.e. a value that may or may not be present. A common use case for optional is the return value of a function that may fail.
What's more, std::optional doesn't need to allocate any memory on the free store. std::optional is a part of C++ vocabulary types along with std::any , std::variant and std::string_view .
C++17 has extended existing if statement’s syntax. Now it is possible to provide initial condition within if statement itself. This new syntax is called "if statement with initializer". This enhancement simplifies common code patterns and helps users keep scopes tight.
This new syntax is called "if statement with initializer". This enhancement simplifies common code patterns and helps users keep scopes tight. Which in turn avoids variable leaking outside the scope.
In C++17 the init statement is called an initializer, and we can directly put it into the if-else block as follows. if (init-statement; condition) { // Do Something } else { // Do Something else }. The scope of the conditioned variable gets limited to the current if-else block.
In C++17 the init statement is called an initializer, and we can directly put it into the if-else block as follows if (init-statement; condition) { // Do Something } else { // Do Something else } The scope of the conditioned variable gets limited to the current if-else block.
There is not, and cannot possibly be, such an Unpack() function.
But you could certainly do:
if (std::optional<int> o = GetOptionalInt(); o) {
// use *o here
}
though the extra o check is kind of redundant.
This is one of those places where it'd be nice if optional<T> modeled a container of at most one element, so that you could do:
for (int value : GetOptionalInt()) {
// possibly not entered
}
but we don't have that interface.
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