Incorrect use of explicit keyword in c++ - c++

Question

I wanted to create a class MPSList where constructor has an explicit keyword associated with it.
Following is the bare minimal code:
class MPSList {
public:
explicit MPSList(int n) : n_(n) {
mpsL.resize(n_, std::vector<MPSNode>{});
std::cout << mpsL.size() << std::endl;
}
private:
struct MPSNode {
double s_;
};
std::vector<std::vector<MPSNode>> mpsL;
int n_ = -1;
};
CPP file that creates the object of MPSList class.
#include <iostream>
#include "MPSList.hpp"
int main() {
double n = 10.9;
MPSList mps(n);
}
On compiling the above CPP file, I had expected to see an error in initializing the object. As I am passing a double whereas the constructor is explicitly expecting an int.
Command to compile:
g++ -std=c++14 -I../include test.cpp
./a.out

Answer 1

Explicit stops the compiler from doing something like this:
void fn(MPSNode x); // or void fn(const MPSNode& x)
fn(3.0);
The above snippet would compile if you didn't use explicit, and the line that calls fn is equivalent to:
fn(MPSNode(3.0));
This is an implicit conversion from double to MPSNode. Narrowing conversions have relatively little to do with it.
However, you will find that the following will not compile:
MPSList mps{n};
Use uniform initialisation syntax if you want to catch problems like that.