I am currently working on an HTTP API that I want to use to perform CRUD operations on a database. I try to write the code for it as generic and modular as possible. I am using the MySQL X DevAPI.
Currently, I am stuck on the following problem:
mysqlx::Result MySQLDatabaseHandler::jsonToCUDOperation (const nlohmann::json& json, mysqlx::Table& table, int crudEnum)
The function above takes as an argument a reference to a json object, a reference to a table object and an integer.
What I want this function to do is:
Check the integer to decide what operation to perform
Check the size of the json to know how many parameters are gonna be passed to the variadic function of the X DevAPI that is used to perform the operation.
Assemble and perform the function call
For example, assume a table "users", as well as a json object "X" with following contents:
{"id":1,"username":"test_user","email":"test#test.com","first_name":"test"}
Now, when I would call the function like this
jsonToCUDOperation(X, users, MySQLDatabaseHandler::cud::create);
I would want the function to parse the json object and call the mysqlx::Table::Insert function with parameters (and parameter count) based on the json object's keys and values, so eventually calling
users.insert("id", "username", "email", "first_name")
.values("1", "test_user", "test#test.com", "test").execute();
I first thought about achieving this behavior using a template function, but then I figured it wouldn't make sense, since the template function definitions are generated at compile time, and what I desire would require dynamic behavior at runtime. So I thought that it is not possible to design this as I intend, as it was my understanding that the behavior of a C++ function cannot change at runtime based on the parameters you pass to it. But I've figured that before I begin developing a solution which can only handle a limited json object size, I'd ask here to assure that I actually cant do what I want.
Thanks in advance for enlightening me
You can actually just pass STL containers to the CRUD functions provided by MySQL's X DevAPI
Related
While doing a game engine that uses .lua files in order to read parameter values, I got stuck when I had to read these values and assign them to the parameters of each component in C++. I tried to investigate the way Unity does it, but I didn't find it (and I'm starting to doubt that Unity has to do it at all).
I want the parameters to be initialized automatically, without the user having to do the process of
myComponentParameter = readFromLuaFile("myParameterName")
for each one of the parameters.
My initial idea is to use the std::variant type, and storing an array of variants in order to read them automatically. My problems with this are:
First of all, I don't know how to know the type that std::variant is storing at the moment (tried with std::variant::type, but it didn't work for the template), in order to cast from the untyped .lua value to the C++ value. For reference, my component initialization looks like this:
bool init(luabridge::LuaRef parameterTable)
{
myIntParameter = readVariable<int>(parameterTable, "myIntParameter");
myStringParameter = readVariable<std::string>(parameterTable, "myStringParameter");
return true;
}
(readVariable function is already written in this question, in case you're curious)
The second problem is that the user would have to write std::get(myIntParameter); whenever they want to access to the value stored by the variant, and that sounds like something worse than making the user read the parameter value.
The third problem is that I can't create an array of std::variant<any type>, which is what I would like to do in order to automatically initialize the parameters.
Is there any good solution for this kind of situation where I want the init function to not be necessary, and the user doesn't need to manually set up the parameter values?
Thanks in advance.
Let's expand my comment. In a nutshell, you need to get from
"I have some things entered by the user in some file"
to:
"the client code can read the value without std::get"
…which roughly translates to:
"input validation was done, and values are ready for direct use."
…which implies you do not store your variables in variants.
In the end it is a design question. One module somewhere must have the knowledge of which variable names exist, and the type of each, and the valid values.
The input of that module will be unverified values.
The output of the module will probably be some regular c++ struct.
And the body of that module will likely have a bunch of those:
config.foo = readVariable<int>("foo");
config.bar = readVariable<std::string>("bar");
// you also want to validate values there - all ints may not be valid values for foo,
// maybe bar must follow some specific rules, etc
assuming somewhere else it was defined as:
struct Configuration {
int fooVariable;
std::string bar;
};
Where that module lives depends on your application. If all expected types are known, there is no reason to ever use a variant, just parse right away.
You would read to variants if some things do not make sense until later. For instance if you want to read configuration values that will be used by plugins, so you cannot make sense of them yet.
(actually even then simply re-parsing the file later, or just saving values as text for later parsing would work)
I am kind of a newbie and I am creating a framework to evolve objects in C++ with an evolutionary algorithm.
An evolutionary algorithm evolves objects and tests them to get the best solution (for example, evolve the weights neural network and test it on sample data, so that in the end you get a network which has a good accuracy, without having trained it).
My problem is that there are lots of parameters for the algorithm (type of selection/crossover/mutation, probabilities for each of them...) and since it is a framework, the user should be able to easily access and modify them.
CURRENT SOLUTION
For now, I created a header file parameters.h of this form:
// DON'T CHANGE THESE PARAMETERS
//mutation type
#define FLIP 1
#define ADD_CONNECTION 2
#define RM_CONNECTION 3
// USER DEFINED
static const int TYPE_OF_MUTATION = FLIP;
The user modifies the static variables TYPE_OF_MUTATION and then my mutation function tests what the value of TYPE_OF_MUTATION is and calls the right mutation function.
This works well, but it has a few drawbacks:
when I change a parameter in this header and then call "make", no change is taken into account, I have to call "make clean" then "make". From what I saw, it is not a problem in the makefile but it is how building works. Even if it did re-build when I change a parameter, it would mean re-compile the whole project as these parameters are used everywhere; it is definitely not efficient.
if you want to run the genetic algorithm several times with different parameters, you have to run it a first time then save the results, change the parameters then run it a second time etc.
OTHER POSSIBILITIES
I thought about taking these parameters as arguments of the top-level function. The problem is that the function would then take 20 arguments or so, it doesn't seem really readable...
What I mean about the top-level function is that for now, the evolutionary algorithm is run simply by doing this:
PopulationManager myPop;
myPop.evolveIt();
If I defined the parameters as arguments, we would have something like:
PopulationManager myPop;
myPop.evolveIt(20,10,5,FLIP,9,8,2,3,TOURNAMENT,0,23,4);
You can see how hellish it may be to always define parameters in the right order !
CONCLUSION
The frameworks I know make you build your algorithm yourself from pre-defined functions, but the user shouldn't have to go through all the code to change parameters one by one.
It may be useful to indicate that this framework will be used internally, for a definite set of projects.
Any input about the best way to define these parameters is welcome !
If the options do not change I usually use a struct for this:
enum class MutationType {
Flip,
AddConnection,
RemoveConnection
};
struct Options {
// Documentation for mutation_type.
MutationType mutation_type = MutationType::Flip;
// Documentation for integer option.
int integer_option = 10;
};
And then provide a constructor that takes these options.
Options options;
options.mutation_type = MutationType::AddConnection;
PopulationManager population(options);
C++11 makes this really easy, because it allows specifying defaults for the options, so a user only needs to set the options that need to be different from the default.
Also note that I used an enum for the options, this ensures that the user can only use correct values.
This is a classic example of polymorphism. In your proposed implementation you're doing a switch on constant to decide which polymorphic mutation algorithm you will choose to decide how to mutate the parameter. In C++, the corresponding mechanisms are templates (static polymorphism) or virtual functions (dynamic polymorphism) to select the appropriate mutating algorithm to apply to the parameter.
The templates way has the advantage that everything is resolvable at compile time and the resulting mutating algorithm could be inlined entirely, depending on the implementation. What you give up is the ability to dynamically select parameter mutation algorithms at runtime.
The virtual function way has the advantage that you can defer the choice of mutation algorithm until runtime, allowing this to vary based on input from the user or whatnot. The disadvantage is that the mutation algorithm can no longer be inlined and you pay the cost of a virtual function call (an extra level of indirection) when you mutate the parameter.
If you want to see a real example of how "algorithmic mutation" can work, look at evolve.cpp in my Iterated Dynamics repository on github. This is C code converted to C++ so it is neither using templates nor using virtual functions. Instead it uses function pointers and a switch-on-constant to select the appropriate code. However, the idea is the same.
My recommendation would be to see if you can use static polymorphism (templates) first. From your initial description you were fixing the mutation at compile-time anyway, so you're not giving anything up.
If that was just a prototyping phase and you intended to support switching of mutation algorithms at runtime, then look at virtual functions. As the other answer recommended, please shun C-style coding like #define constants and instead use proper enums.
To solve the "long parameter list smell", the idea of packing all the parameters into a structure is a good one. You can achieve more readability on top of that by using the builder pattern to build up the structure of parameters in a more readable way than just assigning a bunch of values into a struct. In this blog post, I applied the builder pattern to the resource description structures in Direct3D. That allowed me to more directly express these "bags of data" with reasonable defaults and directly reveal my intent to override or replace default values with special values when necessary.
I'm developing a module that returns a table full of functions based on the arguments that are passed in. Specifically, the module returns a set of data transformation rules (functions) that need to be applied to a data set depending on which customer is sending it.
I decided to decouple my rule library (biz logic) from the code that decides which of the rules should be applied (config logic).
Here's the unit test I'm writing to verify that the ruleBuilder is adding the correct rule (function) based on one of my scenarios:
ruleBuilder = require("ruleBuilder")
ruleLibrary = require("ruleLibrary")
local rules = ruleBuilder.assembleRules("Customer1231")
assert(rules[1] == ruleLibrary.missingSSNRule)
Is this the correct way to do that verification? Will this work even if the ruleLibrary.missingSSNRule function has references to several other functions via a closure or parameter?
To verify that a table contains a particular function you may use the fact that keys in Lua tables can be anything (including functions). In your assembleRules code you can write something like this:
function assembleRules(...)
...
return {
[someOtherCoolModule.coolFunction] = someOtherCoolModule.coolFunction,
[yetAnotherModule.anotherFunction] = yetAnotherModule.anotherFunction,
}
end
Then later you can simply check if the key exists:
local rules = ruleBuilder.assembleRules("somedata")
assert(rules[someOtherCoolModule.coolFunction])
On the assumption that the return value of ruleBuilder.assembleRules is supposed to somehow know to put someOtherCoolModule.coolFunction in the 0-th index (note: Lua uses 1-based indices. Don't use 0 as an index) of its return value, then yes.
Will this work even if someOtherCoolModule.coolFunction is a closure?
All functions in Lua are closures. However, I'm going to assume that you mean that ruleBuilder.assembleRules is going to take someOtherCoolModule.coolFunction and build a new function around it.
A function is equal to itself. But it is only equal to itself. Just like two tables are only equal if they are the same table object, two functions are only equal if they are the same function. Functions are not equal to a different instantiation of the same function, nor is it equal to any other function. Here are examples of this.
I have this view function search(request). The url suffix is /search. It takes a few POST parameters and shows search results accordingly.
I want to make a second function show_popular(request). It takes no post or get parameters. But it should emulate a call to the search function with some hard coded post parameters.
I want to achieve this without changing anything in any existing function and without changing setup. Is that possible?
EDIT: I know this can be achieved by refactoring the search into a separate function and have several view functions call this. But in this particular case, I am not interested in that. In my case the show_popular function is only temporary, and for irrelevant reasons I do not wish to re-factor.
Yes, but you don't want to do that. Refactor search() into a function that handles the request and a function that performs the search, and call the latter from show_popular().
I'm trying to implement the following functionality;
a function with no explicit return will by default return the last evaluation in the last executed block
So, currently the process i'm doing is
1) create a Function
llvm::Function* result = llvm::Function::Create(Compiler::Detail::getAnonymousFunctionSignature(llvmContext),
llvm::GlobalValue::ExternalLinkage,
name,
module());
result->setCallingConv( llvm::CallingConv::C );
2) add blocks and evaluations to the blocks
builder.createFoo.....
However, only in the second phase i have the llvm::Value* (and compile-time type) that i want to use by default as return value. The problem is that i need to use this type to determine the signature of the created function
Question:
how do i solve the problem?
is possible to change the signature after the function is created? is it legal?
do i need to create a new function with the updated signature and copy/assign the entry block of the first function to it and thats it? or do i need to reevaluate all the expressions?
is possible to not create the function before code generation? if it is so, at what point should i create the function?
a code example of how to achieve this would be awesome. thanks!
You cannot change function signature, because this will mean that it will have different Type (and thus you will need to update all the users, etc.; this procedure in most cases cannot be done automagically).
There are multiple possible solutions, for example, you can create the function with the updated signature, then use the functions from lib/Transforms/Utils/CloneFunction.cpp to copy the function body and then hack on the return type.
A better solution exists than CloneFunctionInto(), according to https://stackoverflow.com/a/18751365/2024042:
NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
Where NF is the new function you're cloning into and F is the old function that you have just cloned.