问题
I have the following struct which will be used to hold plugin information. I am very sure this will change (added to most probably) over time. Is there anything better to do here than what I have done assuming that this file is going to be fixed?
struct PluginInfo
{
public:
std::string s_Author;
std::string s_Process;
std::string s_ReleaseDate;
//And so on...
struct PluginVersion
{
public:
std::string s_MajorVersion;
std::string s_MinorVersion;
//And so on...
};
PluginVersion o_Version;
//For things we aren't prepared for yet.
void* p_Future;
};
Further, is there any precautions I should take when building shared objects for this system. My hunch is I'll run into lots of library incompatibilities. Please help. Thanks
回答1:
As was said by someone else, for binary compatibility you will most likely restrict yourself to a C API.
The Windows API in many places maintains binary compatibility by putting a size member into the struct:
struct PluginInfo
{
std::size_t size; // should be sizeof(PluginInfo)
const char* s_Author;
const char* s_Process;
const char* s_ReleaseDate;
//And so on...
struct PluginVersion
{
const char* s_MajorVersion;
const char* s_MinorVersion;
//And so on...
};
PluginVersion o_Version;
};
When you create such a beast, you need to set the size member accordingly:
PluginInfo pluginInfo;
pluginInfo.size = sizeof(pluginInfo);
// set other members
When you compile your code against a newer version of the API, where the struct has additional members, its size changes, and that is noted in its size member. The API functions, when being passed such a struct presumably will first read its size member and branch into different ways to handle the struct, depending on its size.
Of course, this assumes that evolution is linear and new data is always only added at the end of the struct. That is, you will never have different versions of such a type that have the same size.
However, using such a beast is a nice way of ensuring that user introduce errors into their code. When they re-compile their code against a new API, sizeof(pluginInfo) will automatically adapt, but the additional members won't be set automatically. A reasonably safety would be gained by "initializing" the struct the C way:
PluginInfo pluginInfo;
std::memset( &pluginInfo, 0, sizeof(pluginInfo) );
pluginInfo.size = sizeof(pluginInfo);
However, even putting aside the fact that, technically, zeroing memory might not put a reasonable value into each member (for example, there could be architectures where all bits set to zero is not a valid value for floating point types), this is annoying and error-prone because it requires three-step construction.
A way out would be to design a small and inlined C++ wrapper around that C API. Something like:
class CPPPluginInfo : PluginInfo {
public:
CPPPluginInfo()
: PluginInfo() // initializes all values to 0
{
size = sizeof(PluginInfo);
}
CPPPluginInfo(const char* author /* other data */)
: PluginInfo() // initializes all values to 0
{
size = sizeof(PluginInfo);
s_Author = author;
// set other data
}
};
The class could even take care of storing the strings pointed to by the C struct's members in a buffer, so that users of the class wouldn't even have to worry about that.
Edit: Since it seems this isn't as clear-cut as I thought it is, here's an example.
Suppose that very same struct will in a later version of the API get some additional member:
struct PluginInfo
{
std::size_t size; // should be sizeof(PluginInfo)
const char* s_Author;
const char* s_Process;
const char* s_ReleaseDate;
//And so on...
struct PluginVersion
{
const char* s_MajorVersion;
const char* s_MinorVersion;
//And so on...
};
PluginVersion o_Version;
int fancy_API_version2_member;
};
When a plugin linked to the old version of the API now initializes its struct like this
PluginInfo pluginInfo;
pluginInfo.size = sizeof(pluginInfo);
// set other members
its struct will be the old version, missing the new and shiny data member from version 2 of the API. If it now calls a function of the second API accepting a pointer to PluginInfo, it will pass the address of an old PluginInfo, short one data member, to the new API's function. However, for the version 2 API function, pluginInfo->size will be smaller than sizeof(PluginInfo), so it will be able catch that, and treat the pointer as pointing to an object that doesn't have the fancy_API_version2_member. (Presumably, internal of the host app's API, PluginInfo is the new and shiny one with the fancy_API_version2_member, and PluginInfoVersion1 is the new name of the old type. So all the new API needs to do is to cast the PluginInfo* it got handed be the plugin into a PluginInfoVersion1* and branch off to code that can deal with that dusty old thing.)
The other way around would be a plugin compiled against the new version of the API, where PluginInfo contains the fancy_API_version2_member, plugged into an older version of the host app that knows nothing about it. Again, the host app's API functions can catch that by checking whether pluginInfo->size is greater than the sizeof their own PluginInfo. If so, the plugin presumably was compiled against a newer version of the API than the host app knows about. (Or the plugin write failed to properly initialize the size member. See below for how to simplify dealing with this somewhat brittle scheme.)
There's two ways to deal with that: The simplest is to just refuse to load the plugin. Or, if possible, the host app could work with this anyhow, simply ignoring the binary stuff at the end of the PluginInfo object it was passed which it doesn't know how to interpret.
However, the latter is tricky, since you need to decide this when you implement the old API, without knowing exactly what the new API will look like.
回答2:
What about this, or am I thinking too simple?
struct PluginInfo2: public PluginInfo
{
public:
std::string s_License;
};
In your application you are probably passing around only pointers to PluginInfos, so version 2 is compatible to version 1. When you need access to the version 2 members, you can test the version with either dynamic_cast<PluginInfo2 *> or with an explicit pluginAPIVersion member.
回答3:
Either your plugin is compiled with the same version of C++ compiler and std library source (or its std::string implementation may not be compatible, and all your string fields will break), in which case you have to recompile the plugins anyway, and adding fields to the struct won't matter
Or you want binary compatibility with previous plugins, in which case stick to plain data and fixed size char arrays ( or provide an API to allocate the memory for the strings based on size or passing in a const char* ), in which case it's not unheard of to have a few unused fields in the struct, and then change these to be usefully named items when the need arises. In such cases, it's also common to have a field in the struct to say which version it represents.
But it's very rare to expect binary compatibility and make use of std::string. You'll never be able to upgrade or change your compiler.
回答4:
what rwong suggest (std::map<std::string, std::string>) is a good direction. This is makes it possible to add deliberate string fields. If you want to have more flexibility you might declare an abstract base class
class AbstractPluginInfoElement { public: virtual std::string toString() = 0;};
and
class StringPluginInfoElement : public AbstractPluginInfoElement
{
std::string m_value;
public:
StringPluginInfoElement (std::string value) { m_value = value; }
virtual std::string toString() { return m_value;}
};
You might then derive more complex classes like PluginVersion etc. and store a map<std::string, AbstractPluginInfoElement*>.
回答5:
One hideous idea:
A std::map<std::string, std::string> m_otherKeyValuePairs; would be enough for the next 500 years.
Edit:
On the other hand, this suggestion is so prone to misuse that it may qualify for a TDWTF.
Another equally hideous idea:
a std::string m_everythingInAnXmlBlob;, as seen in real software.
(hideous == not recommended)
Edit 3:
- Advantage:
Thestd::mapmember is not subject to object slicing. When older source code copies an PluginInfo object that contains new keys in the property bag, the entire property bag is copied. - Disadvantage:
many programmers will start adding unrelated things to the property bag, and even starts writing code that processes the values in the property bag, leading to maintenance nightmare.
回答6:
Here's an idea, not sure whether it works with classes, it for sure works with structs: You can make the struct "reserve" some space to be used in the future like this:
struct Foo
{
// Instance variables here.
int bar;
char _reserved[128]; // Make the class 128 bytes bigger.
}
An initializer would zero out whole struct before filling it, so newer versions of the class which would access fields that would now be within the "reserved" area are of sane default values.
If you only add fields in front of _reserved, reducing its size accordingly, and not modify/rearrange other fields you should be OK. No need for any magic. Older software will not touch the new fields as they don't know about them, and the memory footprint will remain the same.
来源：https://stackoverflow.com/questions/3930787/is-there-any-way-to-prepare-a-struct-for-future-additions