C++ Lambda causing 'candidate function not viable:'

Question

Why would the following lambda not adhere to the signature when adding a capture element?

[&callback](unsigned handle, void* userData, void* data, unsigned size) -> void { ... }

but this would?

[](unsigned handle, void* userData, void* data, unsigned size) -> void { ... }

Signature:

typedef void (*em_async_wget2_data_onload_func)(unsigned, void*, void*, unsigned);

Answer 1

Lambdas with captures can not be simply converted to function pointers. For maximum flexibility (but not performance) consider the use of std::function<void(unsigned,void*,void*,unsigned)>

Answer 2

Only captureless lambdas can be converted to a pointer to a function.