Init statement for if/switch
New versions of the if and switch statements for C++:
if (init; condition) and switch (init; condition).
Previously you had to write:
{
auto val = GetValue();
if (condition(val))
// on success
else
// on false...
}
Look, that val has a separate scope, without that it 'leaks' to enclosing scope.
Now you can write:
if (auto val = GetValue(); condition(val))
// on success
else
// on false...
val is visible only inside the if and else statements, so it doesn't 'leak.'
condition might be any condition, not only if val is true/false.
Why is this useful?
Let's say you want to search for a few things in a string:
const std::string myString = "My Hello World Wow";
const auto it = myString.find("Hello");
if (it != std::string::npos)
std::cout << it << " Hello\n"
const auto it2 = myString.find("World");
if (it2 != std::string::npos)
std::cout << it2 << " World\n"
We have to use different names for it or enclose it with a separate scope:
{
const auto it = myString.find("Hello");
if (it != std::string::npos)
std::cout << it << " Hello\n"
}
{
const auto it = myString.find("World");
if (it != std::string::npos)
std::cout << it << " World\n"
}
The new if statement will make that additional scope in one line:
if (const auto it = myString.find("Hello"); it != std::string::npos)
std::cout << it << " Hello\n";
if (const auto it = myString.find("World"); it != std::string::npos)
std::cout << it << " World\n";
As mentioned before, the variable defined in the if statement is also visible in the else block. So you can write:
if (const auto it = myString.find("World"); it != std::string::npos)
std::cout << it << " World\n";
else
std::cout << it << " not found!!\n";
Working sample:
Plus, you can use it with structured bindings (following Herb Sutter code):
// better together: structured bindings + if initializer
if (auto [iter, succeeded] = mymap.insert(value); succeeded) {
use(iter); // ok
// ...
} // iter and succeeded are destroyed here
More details in
GCC: 7.0, Clang: 3.9, MSVC: in VS 2017.3.