How to initialize nested struct in C++? - c++

Question

Let's say I have following declaration in the C++:
struct Configuration {
struct ParametersSetA {
float param_A_01;
float param_A_02;
float param_A_03;
} parameters_set_A;
struct ParametersSetB {
float param_B_01;
float param_B_02;
} parameters_set_B;
};
Then somewhere in the code I have following definition:
Configuration config = {
.parameters_set_A = {
.param_A_01 = 1.0f,
.param_A_02 = 2.0f,
.param_A_03 = param_A_01 + param_A_02;
},
.parameters_set_B = {
.param_B_01 = 0.50f,
.param_B_02 = 0.75f
}
};
My question is whether the initialization (especially as far as the param_A_03 item in the nested struct ParametersSetA)
I have used above is correct in the C++?

Answer 1

The problem is that we can't use any of the unqualified names param_A_01 or param_A_02 or an expression that involves any of the two like param_A_01 + param_A_02 as initializer forparam_A_03.
Additionally, you have incorrectly put a semicolon ; after param_A_01 + param_A_02 instead of comma , . I've corrected both of these in the below shown modified program:
Method 1
//create constexpr variables
constexpr float A1 = 1.0f;
constexpr float A2 = 2.0f;
Configuration config = {
.parameters_set_A = {
.param_A_01 = A1,
.param_A_02 = A2,
//--------------vv---vv---->use those constexpr variable(also semicolon is removed from here)
.param_A_03 = A1 + A2
},
.parameters_set_B = {
.param_B_01 = 0.50f,
.param_B_02 = 0.75f
}
};
Working demo
Method 2
Other option is to use qualified names like config.parameters_set_A.param_A_01 and config.parameters_set_A.param_A_02 as shown below:
Configuration config = {
.parameters_set_A = {
.param_A_01 = 1.0f,
.param_A_02 = 2.0f,
//--------------vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv--->qualified name used here
.param_A_03 = config.parameters_set_A.param_A_01 + config.parameters_set_A.param_A_02
},
.parameters_set_B = {
.param_B_01 = 0.50f,
.param_B_02 = 0.75f
}
};
Working demo