Initializer within if and switch statement

To restrict the scope of visibility for a variable which is needed only within an if-block, you can define the variable directly within the if statement. After the if-block the defined variable is unknown.

Principle:

if (init; condition)

Concrete example:

int GetNumFromSomeWhere();
...
// within some function
if (int i = 5; i < 2)
{
    std::cout << "i=" << i << std::endl;
}
else if (auto j = GetNumFromSomeWhere(); j < 0)
{
    // here both i and j are existing
    std::cout << "i=" << i << " j=" << j << std::endl;
}
else
{
    // here also both i and j are existing
    std::cout << "i=" << i << " j=" << j << std::endl;
}

// here both i and j are NOT existing

• use of “auto” within intializer list is possible
• variable declarations are available to all nested if-else sub blocks

Use of structured binding is also possible:

std::tuple<double, double, double> GetSomeCoords()
...
// within some function use structured binding
if (auto [x, y, z] = GetSomeCoords(); (x > 0.05) && (y < 2.2))
{
    std::cout << "coord=" << x << "/" << y << "/" << z << "/" << std::endl;
}

Restriction: You cannot initialize 2 or more variables of different types within a single if-statement (except using structured binding)

Remark: Ugly initialization of multiple values of the same type is possible:
int i=0, j=5;

The same rules for initialization apply also to switch and for statements:

switch (init; var)

for (auto vec = std::vector{1,2,3}; auto v : vec) {/*iterate over vec*/}