C++ 标准特性和二进制大小

Question

I was told recently in a job interview their project works on building the smallest size binary for their application (runs embedded) so I would not be able to use things such as templating or smart pointers as these would increase the binary size, they generally seemed to imply using things from std would be generally a no go (not all cases).

After the interview, I tried to do research online about coding and what features from standard lib caused large binary sizes and I could find basically nothing in regards to this. Is there a way to quantify using certain features and the size impact they would have (without needing to code 100 smart pointers in a code base vs self managed for example).

Answer 1

(Partially extracted from comments I wrote earlier)

I don't think there is a comprehensive answer. A lot also depends on the specific use case and needs to be judged on a case-by-case basis.

Templates

Templates may result in code bloat, yes, but they can also avoid it. If your alternative is introducing indirection through function pointers or virtual methods, then the templated function itself may become bigger in code size simply because function calls take several instructions and removes optimization potential.

Another aspect where they can at least not hurt is when used in conjunction with type erasure. The idea here is to write generic code, then put a small template wrapper around it that only provides type safety but does not actually emit any new code. Qt's QList is an example that does this to some extend.

This bare-bones vector type shows what I mean:

class VectorBase
{
protected:
    void** start, *end, *capacity;

    void push_back(void*);
    void* at(std::size_t i);
    void clear(void (*cleanup_function)(void*));
};

template<class T>
class Vector: public VectorBase
{
public:
    void push_back(T* value)
    { this->VectorBase::push_back(value); }

    T* at(std::size_t i)
    { return static_cast<T*>(this->VectorBase::at(i)); }

    ~Vector()
    { clear(+[](void* object) { delete static_cast<T*>(object); }); }
};

By carefully moving as much code as possible into the non-templated base, the template itself can focus on type-safety and to provide necessary indirections without emitting any code that wouldn't have been here anyway.

(Note: This is just meant as a demonstration of type erasure, not an actually good vector type)

Smart pointers

When written carefully, they won't generate much code that wouldn't be there anyway. Whether an inline function generates a delete statement or the programmer does it manually doesn't really matter.

The main issue that I see with those is that the programmer is better at reasoning about code and avoiding dead code. For example even after a unique_ptr has been moved away, the destructor of the pointer still has to emit code. A programmer knows that the value is NULL, the compiler often doesn't.

Another issue comes up with calling conventions. Objects with destructors are usually passed on the stack, even if you declare them pass-by-value. Same for return values. So a function unique_ptr<foo> bar(unique_ptr<foo> baz) will have higher overhead than foo* bar(foo* baz) simply because pointers have to be put on and off the stack.

Even more egregiously, the calling convention used for example on Linux makes the caller clean up parameters instead of the callee. That means if a function accepts a complex object like a smart pointer by value, a call to the destructor for that parameter is replicated at every call site , instead of putting it once inside the function. Especially with unique_ptr this is so stupid because the function itself may know that the object has been moved away and the destructor is superfluous; but the caller doesn't know this (unless you have LTO).

Shared pointers are a different beast altogether, simply because they allow a lot of different tradeoffs. Should they be atomic? Should they allow type casting, weak pointers, what indirection is used for destruction? Do you really need two raw pointers per shared pointer or can the reference counter be accessed through shared object?

Exceptions, RTTI

Generally avoided and removed via compiler flags.

Library components

On a bare-metal system, pulling in parts of the standard library can have a significant effect that can only be measured after the linker step. I suggest any such project use continuous integration and tracks the code size as a metric.

For example I once added a small feature, I don't remember which, and in its error handling it used std::stringstream . That pulled in the entire iostream library. The resulting code exceeded my entire RAM and ROM capacity. IIRC the issue was that even though exception handling was deactivated, the exception message was still being set up.

Move constructors and destructors

It's a shame that C++'s move semantics aren't the same as for example Rust's where objects can be moved with a simple memcpy and then "forgetting" their original location. In C++ the destructor for a moved object is still invoked, which requires more code in the move constructor / move assignment operator, and in the destructor.

Qt for example accounts for such simple cases in its meta type system .

Answer 2

This question probably deserves more attention than it's likely to get, especially for people trying to pursue a career in embedded systems. So far the discussion has gone about the way that I would expect, specifically a lot of conversation about the nuances of exactly how and when a project built with C++ might be more bloated than one written in plain C or a restricted C++ subset.

This is also why you can't find a definitive answer from a good old fashioned google search. Because if you just ask the question “is C++ more bloated than X?”, the answer is always going to be “it depends.”

So let me approach this from a slightly different angle. I've both worked for, and interviewed at companies that enforced these kinds of restrictions, I've even voluntarily enforced them myself. It really comes down to this. When you're running an engineering organization with more than one person with plans to keep hiring, it is wildly impractical to assume everyone on your team is going to fully understand the implications of using every feature of a language. Coding standards and language restrictions serve as a cheap way to prevent people from doing “bad things” without knowing they're doing “bad things”.

How you define a “bad thing” is then also context specific. On a desktop platform, using lots of code space isn't really a “bad” enough thing to rigorously enforce. On a tiny embedded system, it probably is.

C++ by design makes it very easy for an engineer to generate lots of code without having to type it out explicitly . I think that statement is pretty self-evident, it's the whole point of meta-programming, and I doubt anyone would challenge it, in fact it's one of the strengths of the language.

So then coming back to the organizational challenges, if your primary optimization variable is code space, you probably don't want to allow people to use features that make it trivial to generate code that isn't obvious. Some people will use that feature responsibly and some people won't, but you have to standardize around the least common denominator. AC compiler is very simple. Yes you can write bloated code with it, but if you do, it will probably be pretty obvious from looking at it.