a want to read in a vector of a struct element from a json.
JSON:
A: [
{ "list" :[
"a" : 2,
"b" : 4,
"c" : 9
]
}
my vector is : std::vector< structE > vec;
structE{ "a", "b", "c" }
if (A[i].isMember("list")) // &&
{
auto const list= A[i]["list"];
for (auto i2 = 0u; i2 < list.size(); i2++)
{
vec.push_back(list[i2]["list"]);
}
}
I have this error: matching function for call to 'std::vector ::push_back(const Json::Value&)'
I have this error: matching function for call to 'std::vector ::push_back(const Json::Value&)'
From your source code, it appears that you might be using JsonCpp to handle Json in your C++ code, when dealing with Json::Value objects, for basic types such as int and std::string, you can use Json::Value::asInt() and Json::Value::asString() respectively, however, for structs, you need to manually extract the values and construct an object of the struct with those values:
struct E {
int a, b, c;
};
// ...
E createEFromJsonValue(Json::Value const& obj) {
// Create an object of struct E with values taken from the JSON object
return {
obj.get("a", Json::nullValue).asInt(),
obj.get("b", Json::nullValue).asInt(),
obj.get("c", Json::nullValue).asInt(),
};
}
Then you can do:
// ...
vec.push_back(createEFromJsonValue(list[i2]["list"]));
// ...
Related
I am attempting to translate my Python program to C++, but because I am new to C++ I am encountering some problems. The input file is first parsed (works, not shown) to create the INITIAL_VALUES dict/map, which I then want to use to assign the Parameters class/struct attributes using the DEST_DICT_PARAMS dict/map.
I was able to achieve this in Python code with:
import dataclasses
INITIAL_VALUES = {
"BULK": {
"MAGMA": {
"M0": 1.0,
"T0": 1320.0,
},
"ASSIM": {
"M0": 0.0,
"T0": 600.0,
},
}
}
DEST_DICT_PARAMS = {
'M0': {"MAGMA": 'Mm0', "ASSIM": 'Ma0'},
'T0': {"MAGMA": 'Tm0', "ASSIM": 'Ta0'},
}
#dataclasses.dataclass
class Parameters:
Mm0: float = None
Ma0: float = None
Ta0: float = None
Tm0: float = None
class ParametersReader:
def __init__(self):
self.parameters = Parameters()
self._assignParameters()
def _assignParameters(self):
for param_fam, dest in DEST_DICT_PARAMS.items():
for component, param in dest.items():
value = INITIAL_VALUES["BULK"][component][param_fam]
setattr(self.parameters, param, value)
params = ParametersReader()
print(params.parameters)
Output:
Parameters(Mm0=1.0, Ma0=0.0, Ta0=600.0, Tm0=1320.0)
So I wrote the corresponding C++ code:
#include <iostream>
#include <map>
using std::map;
using std::string;
map<string, map<string, map<string, float> > > INITIAL_VALUES = {{
"BULK", {
{"MAGMA", {
{"M0", 1.0},
{"T0", 1320.0},
}},
{"ASSIM", {
{"M0", 0.0},
{"T0", 600.0},
}},
}
}};
map<string, map<string, string> > DEST_DICT_PARAMS = {{
{"M0", {{"MAGMA", "Mm0"}, {"ASSIM", "Ma0"}}},
{"T0", {{"MAGMA", "Tm0"}, {"ASSIM", "Ta0"}}},
}};
struct Parameters {
float Mm0;
float Ma0;
float Ta0;
float Tm0;
} parameters;
class ParametersReader {
public:
void assignParameters_() {
map<string, map<string, string> >::iterator itr0;
map<string, string>::iterator itr1;
for (itr0 = DEST_DICT_PARAMS.begin(); itr0 != DEST_DICT_PARAMS.end(); itr0++) {
for (itr1 = itr0->second.begin(); itr1 != itr0->second.end(); itr1++) {
parameters.itr1->second = INITIAL_VALUES["BULK"][itr1->first];
}
}
}
};
int main() {
ParametersReader params;
params.assignParameters_();
}
But I'm getting an error at the line
parameters.itr1->second = INITIAL_VALUES['BULK'][itr1->first] saying "no member named 'itr1' in 'Parameters'". That error makes total sense because the code is literally trying to interpret 'itr1' as an attribute name and not the whole 'itr1->second' as the name. I think this comes down to the fact that I can't seem to find a C++ equivalent to Python's setattr(obj, name, val) function that takes an object and its attribute name and assigns it a value. Is there a C++ solution to what I am attempting?
Perhaps my entire approach is incompatible with C++. If so, would you kindly suggest an alternative approach? I would like to keep the input file format the same between the Python and C++ versions.
C++ does not have runtime reflection like Python. You cannot look up a class member by name using a runtime string because class member names do not exist at runtime.
What you can do is look up a class member via a pointer to member. This is an offset into the object calculated at compile time by the compiler:
std::map<std::string, std::map<std::string, float Parameters::*> > DEST_DICT_PARAMS = {{
{"M0", {{"MAGMA", &Parameters::Mm0}, {"ASSIM", &Parameters::Ma0}}},
{"T0", {{"MAGMA", &Parameters::Tm0}, {"ASSIM", &Parameters::Ta0}}},
}};
class ParametersReader {
public:
void assignParameters_() {
for (auto& [param_fam, dest] : DEST_DICT_PARAMS) {
for (auto& [component, param] : dest) {
parameters.*param = INITIAL_VALUES["BULK"][component][param_fam];
}
}
}
};
Demo
Note I've also used range-based for loops and structured bindings to clean up your assignParameters_ function.
C++ has no equivalent to Pythons setattr(self.parameters, param, value). If you want to have a mapping between strings and members you need to write it yourself.
You can use pointers to members to do something along the line of:
#include <map>
#include <iostream>
struct foo {
float a = 0.0f;
float b = 0.0f;
};
// list all members and their string represenation
std::map<std::string,float foo::*> mmap {
{ "a", &foo::a },
{ "b", &foo::b }
};
int main() {
foo f;
std::map<std::string,float> values {
{"a", 0.1},
{"b", 0.2}
};
for (const auto& val : values) {
// lookup the member pointers via the strings from mmap
// use the found function pointer to assing to corresponding member in f
f.*mmap[val.first] = val.second;
}
std::cout << f.a << " " << f.b << "\n";
}
Note that the code assumes that all strings present in values are also present in mmap. If not, it will fail horribly. To fix that mmap.find should be used instead and the case of not found string handleted appropriately.
This works, though there is no way to get that mapping implicitly from the class definition only. On the other, hand I can imagine libraries to exist that can help with that.
TLDR: Is there a way make D's SumType play nice with opCmp while maintaining its functionality?
Context
I'm writing a program for which D's native SumType works almost completely. However, I would like to be able to do the following:
alias Foo = SumType!(int, string);
Foo x = 3;
Foo y = 5;
writeln(max(x, y));
However, since no ordering is natively defined for SumType, I receive the following error:
C:\D\dmd2\windows\bin\..\..\src\phobos\std\algorithm\comparison.d(1644): Error: static assert: "Invalid arguments: Cannot compare types SumType!(int, string) and SumType!(int, string) for ordering."
mwe.d(11): instantiated from here: `max!(SumType!(int, string), SumType!(int, string))`
I was able to remedy this specific issue using the following method:
import std.stdio : writeln;
import std.exception : assertThrown;
import std.algorithm.comparison : max;
import core.exception : AssertError;
import std.sumtype;
struct Foo {
SumType!(int, string) value;
this(T)(T v) {
value = v;
}
ref Atom opAssign(T)(T rhs) {
value = rhs;
return this;
}
int opCmp(Foo other) {
return match!(
(a, b) => a < b ? -1 : a == b ? 0 : 1,
(_1, _2) => assert(0, "Cannot match")
)(value, other.value);
}
}
void main() {
Foo x = 3;
Foo y = 7;
Foo z = "asdf";
assert(x < y); // comparing ints works correctly
assertThrown!AssertError(x < z); // cannot compare int and string
assert(max(x, y) == y); // D's max works
}
The Problem
While I can now use x.value.match!(...) where I used to use x.match!(...), I would like to still be able to call .match! directly on x, and also use match!(...)(x, y) instead of match!(...)(x.value, y.value). I do not like the idea of inserting hundreds of .value throughout my code just to make certain functions like max work, and would prefer if there were a more elegant solution. I tried tinkering around with defining a custom opDispatch using mixins but I couldn't get that to play nicely with the existing SumType:
struct Foo {
SumType!(int, string) value;
this(T)(T v) {
value = v;
}
ref Atom opAssign(T)(T rhs) {
value = rhs;
return this;
}
int opCmp(Foo other) {
return match!(
(a, b) => a < b ? -1 : a == b ? 0 : 1,
(_1, _2) => assert(0, "Cannot match")
)(value, other.value);
}
auto opDispatch(string name, T...)(T vals) {
return mixin("value." ~ name)(vals);
}
}
void main() {
Foo y = 7;
y.match!(
(int intValue) => writeln("Received an integer"),
(string strValue) => writeln("Received a string")
);
}
And I am unable to decode the error which results:
mwe.d(38): Error: none of the overloads of template `std.sumtype.match!(function (int intValue) #safe
{
writeln("Received an integer");
return ;
}
, function (string strValue) #safe
{
writeln("Received a string");
return ;
}
).match` are callable using argument types `!()(Foo)`
C:\D\dmd2\windows\bin\..\..\src\phobos\std\sumtype.d(1659): Candidate is: `match(SumTypes...)(auto ref SumTypes args)`
with `SumTypes = (Foo)`
must satisfy the following constraint:
` allSatisfy!(isSumType, SumTypes)`
Beyond that I am out of ideas as to how to find a less clunky solution.
I suggest giving alias this a try. Similar to class inheritance, this lets you specialize a type and let other things fall back to the original member.
import std.stdio : writeln;
import std.exception : assertThrown;
import std.algorithm.comparison : max;
import core.exception : AssertError;
import std.sumtype;
struct Foo {
SumType!(int, string) value;
this(T)(T v) {
value = v;
}
int opCmp(Foo other) {
return match!(
(a, b) => a < b ? -1 : a == b ? 0 : 1,
(_1, _2) => assert(0, "Cannot match")
)(value, other.value);
}
alias value this;
}
void main() {
Foo x = 3;
Foo y = 7;
Foo z = "asdf";
assert(x < y); // comparing ints works correctly
assertThrown!AssertError(x < z); // cannot compare int and string
assert(max(x, y) == y); // D's max works
// this will now automatically fall back to y.value.match
y.match!(
(int intValue) => writeln("Received an integer"),
(string strValue) => writeln("Received a string")
);
}
See, you still must construct your special type, but then after that, it will look up there for members. It will find the opCmp, letting it extend the type. But then for everything else, since it isn't there, it will try checking obj.value instead, falling back to the original type.
This doesn't always work, and it means it will implicitly convert too, meaning you can pass a Foo to a void thing(SumType!(int, string)) with it passing foo.value to the function, which may or may not be desirable.
But I think it is the closest thing to what you want here.
(note btw why you got an error originally is that match isn't actually a member of SumType. it is an outside free function that takes all the match lambdas as template arguments. An opDispatch could forward template arguments too - it can be done in a two-level definition - but since match is not a member anyway, it isn't quite going to solve things anyway whereas the alias this does seem to work)
I am initializing a shared_ptr to map in a separate function called GetData. This map is passed as argument to GetData function. However, in the main, map comes back empty after the call to GetData function.
#include <memory>
#include <map>
void GetData(std::shared_ptr<std::map<int, int>> data);
int main()
{
std::shared_ptr<std::map<int, int>> data2 = std::make_shared<std::map<int, int>>();
GetData(data2);
return 0;
}
void GetData(std::shared_ptr<std::map<int, int>> data)
{
data = std::make_shared<std::map<int, int>>
(std::initializer_list<std::map<int, int>::value_type>{
{ 1, 2 },
{ 5, 6 },
{ 4, 5 }
});
}
What am I doing wrong here?
UPDATE:
If I re-write method as follows while parameter still not passed by reference, I do get map populated in the main after call to GetData method.
void GetData(std::shared_ptr<std::map<int, int>> data)
{
data->insert(std::pair<int, int>(1,2));
data->insert(std::pair<int, int>(45, 2));
}
Pass data by reference:
void GetData(std::shared_ptr<std::map<int, int>>& data)
Or preferably, so it makes more sense as function that "gets" and returns something, have the value returned
std::shared_ptr<std::map<int, int>> GetData()
{
return std::make_shared<std::map<int, int>>
(std::initializer_list<std::map<int, int>::value_type>{
{ 1, 2 },
{ 5, 6 },
{ 4, 5 }
});
}
Wondering if it's possible to extract the name of a rapidjson::Value directly from it.
For instance, assume we have the following JSON data:
{
"name":
[
{ /*some data*/ },
{ /*some more data*/ }
]
}
And I retrieve the "name" array from it:
rapidjson::Value& myJSONArray = document["name"];
Can I retrieve "name" back from that Value? Something like this:
std::string memberName = myJSONArray.GetMemberName(); // returns "name"
No. It is not possible because an array may not be within an object.
You may use iterator.
Value::MemberIterator itr = document.FindMember("name");
string n = itr->name.GetString();
Value& v = itr->value;
Iterators for object has name and value properties
std::pair<bool, std::string> iterate_items()
{
constexpr std::string_view stringJson = R"([ {"k1": "v1"}, {"k2": "v2"}, {"k3": "v3"}, {"k4": "v4"} ])";
// Wrap input stream for rapidjson reading
rapidjson::MemoryStream memorystreamFile( stringJson.data(), stringJson.length() );
rapidjson::Document documentJson; // Create root rapidjson object
documentJson.ParseStream( memorystreamFile ); // Parse json file
if( documentJson.IsArray() == true ) // Yes, we know it is an array :)
{
for( auto const& it : documentJson.GetArray() ) // iterate array
{
if( it.IsObject() == true ) // They are all objects
{
auto const& _name = it.MemberBegin()->name; // get name
auto const& _value = it.MemberBegin()->value; // get value
std::cout << _name.GetString() << _value.GetString() << "\n"; // dump it
}
}
}
return std::pair<bool, std::string>( true, std::string() );
}
Tutorial with RapidJSON
I'm not that good in English, that's why my Question is probably wrong. But I have a problem and I don't know how to solve it or if it's even possible to do.
I have 2 Structs defined:
typedef struct
{
UINT16 ScriptNumber;
std::string ScriptText;
} StepStruct;
typedef struct
{
std::string SequenceName;
std::string DisplayName;
StepStruct SequenceSteps;
} SequenceStruct;
As you can see, the first Struct is a Member of the second struct. So I want both structs to by dynamical. So I created 2 Dynamic Arrays from the Type StepStruct and 1 dynamic Array from the Type SequenceStruct.
The two dynamical Arrays for of the Type StepStructs are defined as follows:
StepStruct gsFirstSkript[] =
{
{ 1 , "SkriptText One"},
{ 2 , "SkriptText Two"},
{ 45, "SkriptText Three"}
}
StepStruct gsSecondSkript[] =
{
{ 48, "SkriptText One"},
{ 2 , "SkriptText Two"},
{ 45, "SkriptText Three"}
}
Those to Structs are of the Type StepStruct. Now I want to do the Same with a SequenceStruct Type, but I want to assign the two Arrays I already have to it under the Struct Member SequenceSteps. I mean this as follows:
SequenceStruct gsSequenceList[] =
{
{ "FirstScript", "Test One", gsFirstSkript},
{ "SecondScript", "Test Two", gsSecondSkript}
}
If I now want to Read the Member gsSequenceList, I can not access any information under the SequenceSteps Index of it! What means, that the Data is not copied! I tried it with Pointers
but had no success.
UINT16 lTestVal = gsSequenceList[0].SequenceSteps[2].ScriptNumber;
So Can I mangage that this works, and lTestVal contains the Value 45?
typedef struct
{
std::string SequenceName;
std::string DisplayName;
StepStruct* SequenceSteps;
} SequenceStruct;
This will allow the code to compile and the test fragment you've shown will work.
However this will not copy the data. If you change gsFristSkript it will change in gsSequenceList as well. If you want to make a copy of the data you can either do that explicitly, have a constructor or just use vector<>.
Here's the solution with vector:
#include <vector>
...
typedef struct{
std::string SequenceName;
std::string DisplayName;
vector<StepStruct> SequenceSteps;
} SequenceStruct;
vector<StepStruct> gsFirstSkript =
{
{ 1 , "SkriptText One"},
{ 2 , "SkriptText Two"},
{ 45, "SkriptText Three"}
}
vector<StepStruct> gsSecondSkript =
{
{ 48, "SkriptText One"},
{ 2 , "SkriptText Two"},
{ 45, "SkriptText Three"}
}
SequenceStruct gsSequenceList[] =
{
{ "FirstScript", "Test One", gsFirstSkript},
{ "SecondScript", "Test Two", gsSecondSkript}
}