Why is 'mutable' a lambda function attribute, instead of being a capture type?

Question

Why does c++11 require us to write:

[a,b]() mutable { a=7; } // b is needlessly mutable, potential source of bugs 

Instead of:

[mutable a,b]() { a=7; } // no problems here 

Is this an oversight, considered not important enough, or is there a specific technical reason?

Answer 1

There is a mention about your suggestion in n2651:

The syntax for lambda expressions could be extended to allow declaring whether the closure members should be declared mutable or not.

This approach could be confusing to programmers, as the mutability is not a property of the closure object, but rather the variables stored in the closure.

I don't know if this is the only reason, but it does seem like it was considered. However, in Herb Sutter's proposal, he suggests getting rid of mutable and not making the capture copies implicitly const, so we might see changes again.

Answer 2

Probably an oversight (in the same way as rvalue refs can't be used) of sorts and an artifact of the way lambdas are conceptually implemented.

int   a; int*  b; float c;  auto lambda1 = [&a, b, c](int d) mutable -> void {};  class lambda1 { public:     void operator()(int d) {} private:     int&  a_;     int*  b_;     float c_; };  auto lambda2 = [&a, b, c](int d) -> void {};  class lambda2 { public:     void operator()(int d) const {} private:     int&  a_;     int*  b_;     float c_; };