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libcamera: base: utils: Provide defopt to simplify std::optional::value_or() usage

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The std::optional<T>::value_or(U &&default_value) function returns the
contained value if available, or default_value if the std::optional has
no value. If the desired default value is a default-constructed T, the
obvious option is to call std::optional<T>::value_or(T{}). This approach
has two drawbacks:

- The \a default_value T{} is constructed even if the std::optional
  instance has a value, which impacts efficiency.
- The T{} default constructor needs to be spelled out explicitly in the
  value_or() call, leading to long lines if the type is complex.

Introduce a defopt variable that solves these issues by providing a
value that can be passed to std::optional<T>::value_or() and get
implicitly converted to a default-constructed T.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Umang Jain <umang.jain@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
This commit is contained in:
Laurent Pinchart 2022-07-27 23:51:17 +03:00
parent f6d6181d3c
commit 48c106429a
3 changed files with 89 additions and 0 deletions
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54
test/utils.cpp
View file

@ -7,6 +7,7 @@
#include <iostream>
#include <map>
#include <optional>
#include <sstream>
#include <string>
#include <vector>
@ -169,6 +170,55 @@ protected:
return TestPass;
}
int testDefopt()
{
static bool defaultConstructed = false;
struct ValueType {
ValueType()
: value_(-1)
{
defaultConstructed = true;
}
ValueType(int value)
: value_(value)
{
}
int value_;
};
/*
* Test that utils::defopt doesn't cause default-construction
* of a ValueType instance when value_or(utils::defopt) is
* called on a std::optional that has a value.
*/
std::optional<ValueType> opt = ValueType(0);
ValueType value = opt.value_or(utils::defopt);
if (defaultConstructed || value.value_ != 0) {
std::cerr << "utils::defopt didn't prevent default construction"
<< std::endl;
return TestFail;
}
/*
* Then test that the ValueType is correctly default-constructed
* when the std::optional has no value.
*/
opt = std::nullopt;
value = opt.value_or(utils::defopt);
if (!defaultConstructed || value.value_ != -1) {
std::cerr << "utils::defopt didn't cause default construction"
<< std::endl;
return TestFail;
}
return TestPass;
}
int run()
{
/* utils::hex() test. */
@ -281,6 +331,10 @@ protected:
if (testDuration() != TestPass)
return TestFail;
/* utils::defopt test. */
if (testDefopt() != TestPass)
return TestFail;
return TestPass;
}
};