Example
For lambdas with a single return statement, or multiple return statements whose expressions are of the same type, the compiler can deduce the return type:
// Returns bool, because "value > 10" is a comparison which yields a Boolean result
auto l = [](int value) {
return value > 10;
}
For lambdas with multiple return statements of different types, the compiler can't deduce the return type:
// error: return types must match if lambda has unspecified return type
auto l = [](int value) {
if (value < 10) {
return 1;
} else {
return 1.5;
}
};
In this case you have to specify the return type explicitly:
// The return type is specified explicitly as 'double'
auto l = [](int value) -> double {
if (value < 10) {
return 1;
} else {
return 1.5;
}
};
The rules for this match the rules for auto type deduction. Lambdas without explicitly specified return types never return references, so if a reference type is desired it must be explicitly specified as well:
auto copy = [](X& x) { return x; }; // 'copy' returns an X, so copies its input
auto ref = [](X& x) -> X& { return x; }; // 'ref' returns an X&, no copy
