Is there a way to check whether C++ lambda functions are inlined by the compiler?

TL;DR: Not without looking at the compilation output.

First, as other answers point out, C++ lambdas are basically anonymous classes with an operator() method; so, your question is no different than "is there a way to check that a certain invocation of an object's method gets inlined?"

Whether your method invocation is inlined or not is a choice of the compiler, and is not mandated by the language specification (although in some cases it's impossible to inline). This fact is therefore not represented in the language itself (nor by compiler extensions of the language).

What you can do is one of two things:

• Externally examine the compilation output (the easiest way is by compiling without assembling, e.g. gcc -S or clang++ -S; altough inlining could still happen at link-time, theoretically)
• Internally, try to determine side-effects of the inlining choice. For example, you could have a function which gets the address of a function you want to check; then you read - at run-time - the instructions of that function, to see whether it has any function calls, look up the called addresses in the symbol table, and see whether the symbol name comes from some lambda. This is already rather difficult, error-prone, platform-specific and brittle - and there's the fact that you might have two lambda used in the same function. So I obviously wouldn't recommend doing something like that.