user2015453 user2015453 - 3 years ago 154
C++ Question

Is my book's discussion of lambda return types wrong?

My book says this:

Lambdas with function bodies that contain anything other than a single return statement that do not specify a return type return void.

but this:

auto f = []{
int i=0; i++;
return std::string("foo");
std::cout << f() << std::endl;

actually compiles and prints out "foo", but that lambda expr has more than just a single return statement so it should return void, because it does not manually specify "-> std::string" as a return type.

What's going on here?

I'm using Apple's compiler in the latest Xcode 4.6, based on Clang 3.2 it seems:

clang --version

Apple LLVM version 4.2 (clang-425.0.24) (based on LLVM 3.2svn)
Target: x86_64-apple-darwin12.2.0
Thread model: posix

Answer Source

The book accurately reflects the rules in draft n3290 of the Standard. Perhaps your compiler implemented a different draft.

In section 5.1.2p4, the draft reads

If a lambda-expression does not include a trailing-return-type, it is as if the trailing-return-type denotes the following type:

  • if the compound-statement is of the form { attribute-specifier-seqopt return expression ; } the type of the returned expression after lvalue-to-rvalue conversion, array-to-pointer conversion, and function-to-pointer conversion;
  • otherwise, void.

The syntactic construct attribute-specifier-seq may be alignas or the double-bracketed attributes. Not variable declarations.

Draft n3485, which followed publication of C++11 (i.e. it is work in progress toward C++1y), contains the same wording. I don't know if there was a different rule in some draft earlier than n3290.

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