# 重拾 C 語言::函式指標

## 進入正題

void (*foo)();

int (*ftpr1)(int);

int n = 5;

int square(int num){
    return num*num;
}

ftpr1 = square;

printf("%d",ftpr1(n))

其中 `ftpr1 = square;` 其實被精簡過了。原本應該長這樣:

ftpr1 = □

### 傳入函式指標

int add(int num1, int num2){
    return num1 + num2;
}

int sub(int num1, int num2){
    return num1 - num2;
}

typedef int (*ftprOperation)(int, int);

int compute(ftrOperation operation, int num1, int num2){
    return operation(num1, num2);
}

compute(add, 2, 3);
compute(sub, 2, 3);

### 傳回函式指標

既然能夠傳入函式指標，我們同樣也可以利用函式回傳函式指標增加程式的彈性。

fptrOperation select(char opcode){
    switch(opcode){
        case '+': return add;
        case '-': return sub;
    }
}

fptrOperation select(char opcode, int num1, int num2){
    fptrOperation operation = select(opcode);
    return operation(num1, num2);
}

printf("%d\n", evaluate('+', 5, 6));
printf("%d\n", evaluate('-', 5, 6));

### 使用函數指標的陣列

typedef int (*operation)(int, int);
operation operations[128] = {NULL};

若不使用 `typedef` ，可以使用:

int (*operations[128])(int, int) = {NULL};

ASCII 的範圍剛好是從 0 - 127 ，一共128個，我們可以加以利用:


我們對先前的範例稍加修改:

typedef int (*operation)(int, int);
operation operations[128] = {NULL};

int add(int num1, int num2){
    return num1 + num2;
}

int sub(int num1, int num2){
    return num1 - num2;
}

void initOpArray(char opcode){
    switch(opcode){
        case '+': operations['+'] = add;
        case '-': operations['-'] = sub;
    }
}

int evaluateArray(char opcode, int num1, int num2){
    operation doSomething;
    doSomething = oprations[opcode];
    return doSomething(num1, num2);
}

> declare a as array of pointer to function returning pointer to function returning pointer to char

char *(*(*a[])())()