Specifying the return type

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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

Found a mistake? Have a question or improvement idea? Let me know.

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